I've only been to last year's PAX and MAGFest. In and of themselves, they weren't that thrilling. I had the most fun just from the company I had with me, but the concerts at both were really slick. If anybody is going to PAX this year, do make sure you see The OneUps perform, at the very least. For the size of PAX, the Exhibitioner's Hall didn't excite me much. Too many no-name companies and stuff that doesn't even remotely relate to video games. A lot of big-name developers were completely absent, and those that were there didn't show anything really newsworthy. Mostly just games that were already out or due to come out in a week or two. Not much neat promo stuff either. There really weren't any panels worth attending, and the biggest celebrity they had was Wil Wheaton. In terms of retro games, there was a pretty neat little room with a bunch of systems hooked up and games available to play, which I think was arranged by some members here, but it was mostly for people just starting to recapture their childhood gaming. If you already have a huge collection like me, it's nothing you don't see everyday.
As for MAGFest, it's a small-time convention created by fans for fans. I think they have, what, about a thousand people that attend? It's more about a social gathering than exhibits and such. They have a huge gaming hall with classic and modern games, though, and a fair amount of arcade cabinets set on free play scattered around. There's a big focus on music, so if you're into video game music, there are plenty of interesting panels and great shows.