Halo 3 made me go back and buy (but not play) Halo 1 and Halo 2. I was thoroughly impressed. Still, Halo fans said that it was a disappointment for them. It's still huge on XBL, though.
Metal Gear Solid 4 is the follow up to my favorite game series ever (MGS3 > ALL), but if I said I wasn't let down, I'd be lying. This was the single reason for my PS3 purchase.
I guess both games have their pros and cons, but I'm a little more of a Metal Gear fan. I've been playing the series for twenty years!! Will Halo be around that long?