Games, games, games.
It's all about the games. Don't buy a system because of what the system can do. Buy a system because of what the system can play! Remember, all those specs are just potential until someone comes along and actually uses them. And usually by the time that happens, a next generation system comes along anyway.
For me, it was a simple choice of picking the Nintendo GameCube over all others. Not because it had any superior hardware (which it doesn't), but because it had many games that I could only find on the GC. Metroid Prime, Zelda:WW, Star Fox, a host of original RE games, plus you could still play all the games that are out for all systems. As a bonus, I later found out that it could connect with my GBASP to unlock special features in certain games, making games even more fun.
This is exactly what I was looking for in a system, and Nintendo provided for my needs. Whether you're of the same opinion, only you can answer, but just don't look at a system and think "Well, system A can run 433 MHz faster than system B can, therefore system A will obviously be a far better choice". You could end up being sorely disappointed later on when you end buying a $300+ paperweight, and nothing to play on it.
If you have any doubts, just go to the local video rental, or video game store, and look over the titles that are available for all the systems. It'll help you make a better informed decision, and who knows, you might be surprised at what you find out.
Edit: Incidentally, that "System A vs System B" example was real. The XBox was System A, and PSX2 was System B, and we all know who's outselling who.
