First, lol, that avatar. XD
Second, it totally depends on the game. Mortal Kombat and Street Fighter have been both, and I find them being better as 2D/2.5D games, where as Tekken and Soul Calibur works great as 3D games. The freedom you have to move around in Soul Calibur is crucial to its gameplay, and though Street Fighter X Tekken was fun (imo), I didn't like the Tekken characters in 2.5D, even if they did fight as Street Fighter characters. It just didn't feel right.
Overall, I like both, but I think I am leaning mostly towards 2.5D games. Street Fighter, Mortal Kombat, Killer Instinct and Guilty Gear are amongst my favourite franchises. But as said, I like both, since Soul Calibur and Tekken, and to some extend Dead or Alive, are up there as my favourites as well.