Short answer: DiRT 3.
I have/had both games at one point, beat them both, got the platinum trophy on both. MS:A is straight up arcade, floor it, always press boost and never look back. It's fun, looks great, but in the end it just didn't hold any replay value for me. Once I beat the campaign I didn't really want to play it again. DiRT 3 has a lot more replay value, to me, since there's more to do both offline and online.
MS:A had some big online issues, and has been partially fixed. First off, there isn't many online. Now, DiRT 3 has a similar problem to that too, but to me DiRT 3 has more offline replayability if nobody is online. But MS:A also has connection issues. Even if you have great internet, you'll still get "kicked" every once in awhile. I usually got booted once an hour, sometimes more, sometimes less. Some people couldn't even get through a race when it came out, but I understand it is better now.
Graphics wise, MS:A does look pretty good. The hurricane track has great water effects and there are some sweet apocalyptic buildings falling.
Thing is, with MS:A, it just felt like I was playing a better-looking, possibly more fun version of Split/Second. It was like S/S, except the game changes routes for you. It's still cool though.
Like I said before, DiRT 3 is a better game, more variety, more replay value and better overall for the money (compared to MS:A, imo). I sold Apoc awhile ago for about 30-35, and I don't really miss it. It has some great things going for it, but to me, it's really only worth 15 or 20 bucks. I think a lot of work got put into the graphics, high framerate, etc. and in the end the gameplay and online functions suffered a bit. So as I said at the top, I would recommend you go with DiRT 3.
