The smartypants answer is 'both', but you'll probably want the PS2 version. It doesn't struggle with the slowdown present in the GBA version, it's got unlockable commercials (Phil Hartman!) and ephemera, it's got kicky 80's music rockin' while you play -- the only thing missing is six or seven homebrew games (AtariAge.com material) that the GBA version packs in. Honestly though, of those seven only Oystron is really worth coming back to much.
I think the slowdown would bug me. I played these games in their original incarnations so much and so often that any deviation from the real thing is going to stick out like a sore thumb to me. Plus I think the TV would be better than the GBA in this one case and I like the idea of the extras.
Kinda prefer the GBA version. Something just fits playing these games on a portable. 20 years ago if you told me I could play these games on the go, I would had laugh. One of those hoarse puberty laughs twelve year olds have.
That said, I believe the PS2 version has the awesome iMagic games (is that company's name? I forgot), Demon Attack and such.
The GBA version is pretty nice, but it's the presentation on the PS2 version that makes it way better. I really wish the Midway collections were put together that nicely as I prefer those games, etc.
