Defiantly depends on the style of the game.
If its trying to be realistic with real world scenarios then defiantly class based (Battlefield/Call of Duty).
However if its not taking itself as a realistic shooter and has power-ups etc, (ala Quake 3, Unreal Tourny, Timesplitters) then I'm going for power ups.
I can't imagine playing Quake with set weapon system, just because the weapons aren't balanced in the way that your supposed to have just one/two. There would one kit that would dominate. (Off topic: I'm imagining a railgun/shotgun combo. Anyone got a counter for that? maybe LG instead of SG...)
Just as in Call of Duty if you found all the weapons in the game, it wouldn't completely ruin the gameplay, but it would be dramatically different, in a way that would lose immersion, which isn't what the game is about.
Depends on the style of game, it works for some but not for others. not sure which I prefer, I love games that use both methods.
