GURPS, and by extension FATAL, are both games for people who prefer to see what hell their dice rolls unleash with a character they spent a week writing up.

The appeal of the systems are very much the math and comprehensiveness of what is there. 3.5 succeeds in providing an efficient system, but it is also heavily flawed and there are things not covered in the game. 4th provides an even more efficient system, but at the cost of alot of what made 3.5 3.5 (the coke Variety of what you can do and specialize in).

Basically, each system has its own niche for appeal.
4th provides a much more stable and balanced game
3.5 gives alot of options without being itself absurdly complex
GURPS gives an extremely rewarding experience for alot of effort, but there is alot of flexibility in it
Fatal even moreso