My perspective is someone whose first RPG was The Fantasy Trip, and who has played GURPS (mostly home-brew ancient/medieval fantasy settings) ever since, and who still likes TFT and has run my original TFT campaign in GURPS, and who has done a little with the DF RPG, but doesn't much like the D&D-like setting or the super-hero flavor.
The Fantasy Trip's combat system can be learned very quickly, even by children. The very short basic Melee book has most of the combat system in it. It is a fun arena combat game. It's GURPS equivalent is/was Man To Man, but Man To Man is more advanced and complicated than TFT's Advanced Melee was.
If I had to make just one point about TFT, though:
TFT offers a fun, logical, interesting hex mapped tactical combat system, that is very like GURPS Advanced tactical combat, BUT very fast and simplified and accessible to new players.
And, since the logical hex-mapped tactical combat is the main unique thing I think makes TFT & GURPS stand out as great games to play, I think this is the main thing to bear in mind.
We played TFT campaigns heavily from 1980-1986. We looked at D&D from a TFT perspective and thought it was a complete joke because it's so arcane and makes so little sense by comparison. Contrary to a previous comment TFT DOES have a setting (Cidri), and I would say it's actually more developed in terms of usable ready-to-play details than most GURPS world books (which isn't saying much). BUT, it does kind of expect the GM will fill in details, add geography and adventures, but unlike 1980s TFT, there are already now quite a few adventure modules for GMs who don't want to invent their own stuff, and more coming.
My RPG friends and I are into detailed tactical combat, and after 5-6 years of playing TFT, we were ready for something more detailed and realistic, and that was exactly what we found in GURPS Man To Man. I still relate to GURPS as Advanced TFT. And having mastered TFT, GURPS advanced tactical combat was very easy and natural to learn and run.
Now, as for GURPS Dungeon Fantasy, well, with my background, I have a very hard time understanding how any people play it without using hex-mapped combat. To me that's like eating a hamburger without any meat or even without any meat-substitute. But comparing it to GURPS Dungeon Fantasy WITH hex-mapped tactical combat, the main differences I see are:
* Combat system complexity / detail level.
* Character complexity level.
* Magic System.
* PC Power Level
* Genre Expectations
* Deadliness Expectations
* Healing Magic
* Combat system complexity / detail level.
TFT combat is a LOT simpler and faster to learn. Once you learn GURPS combat, though, an experienced GM can run it about as easily and almost as quickly.
The overall results may be largely the same, however, and new players may not appreciate the differences.
TFT combat has engagement which results in melee fighters stopping to fight each other. GURPS combat can be much more fluid and chaotic.
TFT combat has very limited defenses. GURPS combat, especially DF combat, tends to have lots of parries, blocks and dodges.
GURPS has many more options for what to do and how to do it combat, and is blow-by-blow one-second turns instead of TFT 5-second turns.
* Character complexity level.
TFT characters are VERY easy and fast to make, and have rather less detail, much fewer options, and no Disadvantages.
GURPS characters CAN be not ALL that much more complex, but modern 4e examples and DF examples tend to be much denser and more detailed. A lot of that detail can be noise (oh really? what ARE the social disadvantages of ZOMBIES? Please list their official metabolic status. Oh are wolves Quadrupeds? Better write that down and add in the point cost!), though that can be ignored.
* Magic System.
Like the combat system, TFT's magic system is also like a simpler ancestor of GURPS Magic. However it does not suffer from many of the things some GURPS players complain about the GURPS Magic system. TFT has fewer spells, more of them are combat-oriented, and wizards know fewer of them. They do use ST-based fatigue to cast them, but you can eventually get powerstones or mana to help, or use apprentices' ST, like in GURPS Magic.
The set of useful spells and their power levels is a bit different. However many/most/all of the spells in TFT have GURPS Magic analogs.
TFT only has one spell list, and it's divided up only by IQ level prerequisite, not by type of spell or type of caster.
* PC Power Level
GURPS Dungeon Fantasy characters are often expected to start out at heroic levels, and get even more heroic with experience. TFT starting characters are just slightly better than average (32 attribute total rather than population average 30, so in GURPS 4e that's maybe +40 points above average people).
GURPS Dungeon Fantasy characters are also D&D-like heroic archetypes with access to heroic larger-than-realistic abilities, such as Heroic Archer, Ridiculous Luck, or Regeneration. TFT characters may be wizards or eventually very capable fighters, but they don't start sprouting D&D-like super-powers.
* Genre Expectations
TFT's Cidri is a fantasy setting, with fantastic creatures and magic and heroes, but it's a logical one. The guidelines for stocking labyrinths to explore mention that the GM should probably think of why the creatures and traps are where they are, how that makes sense and is possible. It also suggests the game world be mapped out on a hex map with terrain that has affects on movement rates and getting lost and so on, and describes towns, nations, and political situations that look like they make plausible sense. The professions of the world are also things that mostly makes sense.
By contrast, Dungeon Fantasy's reasons for things existing seems mainly usually to be because of D&D tropes. There are "Bards" which are super-hero-like wizards-with-musical-instruments, and who have a guild for that, because it's a D&D trope. Also "Druids", "Scouts", and other classes which have guilds of the same type, not because that makes sense, but because ... well I don't know why, but it doesn't seem to me to be logic-based.
I won't try to enumerate the differences, but what I notice is TFT seems to me fantastic but trying to make some sense, and more "its own thing" rather than trying to copy the forms of another game tradition.
* Deadliness Expectations
TFT expects there to be real likelihood of PCs dying even in ordinary encounters, and it mostly provides that with its combat system and power level suggestions.
The TFT campaign book suggests players may have multiple PCs so there can be losses and replacements, and so some can rest serious wounds or do other things while others adventure.
Dungeon Fantasy can do that too, but it has a stronger assumption players will have single PCs and GMs will conspire to try to make the universe not "too" likely to get them killed.
* Healing Magic
TFT has no magic spells that heal damage. It has healing potions that heal one point of damage, and cost $150, and may not be available. Healers heal 2 or 3 points, beyond which a wound will need 2 days' bed rest per point.
TFT characters die when their ST is reduced below 0, even by one point, unless something (see above) raises them to 0 or more within an hour.
Dungeon Fantasy includes healing spells from GURPS Magic, which are very powerful at healing people, and there are also other healing talents and potions and so on.
GURPS characters don't die until they reach ST x -1, and then they get a roll to avoid dying that heroes tend to be likely to make, etc.
i.e., It's a lot easier to die in TFT than in Dungeon Fantasy. It's also very likely that on prolonged adventures, PCs will actually have to manage and care a lot about how injured their people are, and try to get them to safety where they can rest safely, etc.