Don't buy an Arcade, I bought it and regretted it almost instantly. Have an elite and PS3 and they're both fine, don't really notice the difference. Comes down to where your friends are playing really.
They're both pretty much the same. Only reasons I have an xbox is because I prefer the controllers, it's cheaper and more of my mates have them. The online is exactly the same.
I've always found the opponents easier playing PS3 online than playing on Xbox. Not sure if that's just random, or because with the PS3 online being free you get more 'casual' gamers. And I'm worse at most games on PS3 compared to 360. It's hardly conclusive or anything, but worth considering if you want to pick a challenge or just the fun of winning.