I'd say; go for X, even if you have the previous two Metal Slugs on CD. The game has a more "hard-core" feel to it than 3.I own both. In terms of graphics 3 is superiour, not that the animation is any better; there are just many more enemies than before.Some of the new enemies are brilliant, like the zombies of level 2 and the huge flying things from level 1. Sound effects are Superb, but not better than X's:the same.
It may sound strange, but the worst part of ANY MS game is the upward scrolling spaceshooter bit in the final level. I hate that bit; looks awful and is too long. I know people have been saying things like "WOW, what an amazing addition to the traditional side-scrolling game!!Superb!" Well, I think it sucks.
Oh, and the last level is just waaaaay too long. And the ending sucks, as in that it's not as good as Metal Slug 1.