It all depends what you're into. If you're into light gaming and don't plan on playing competitively and whatnot, go for the Arcade. If you're more of a serious gamer and like playing online, I would go for the Pro 20 GB or 60 GB. It all depends how much you will download and if you will be downloading games onto your hard drive or not. And lastly, the Elite. This is the console I have, and I believe it is the best, if you have the money to pay for it. It is great because of the 120 GB hard drive which can fit A LOT of stuff, and it is great for serious, competitive gamers like myself.
Depends, if you plan on downloading a lot the Elite is the most cost effective. But I'd stay away from the Arcade considering you don't get a hard drive at all and the 360's aren't as hard drive friendly as the PS3. 360's have specially made hard drives so you can't just get any hard drive from a Best Buy or something and swap em out. So if you don't download much and just want a 360 to enjoy games with, I'd go with the Pro 20gb or 60gb whichever is fine.