Not if it's with 2 players - things just don't flow.
I've always enjoyed both, and the slowdown in MS2 doesn't bother me at all. However, X is the better game for me, as it really is 2 with more. There are more weapons, hidden items (for points), better music arrangement, and their are more animations. I love X and have spent far too many hours on it in the past, and for me it is the best Slug in the series.
So, yeah, X is better than 2... and slowdown doesn't come into it for me at all!
X>1><2>3>5>>>>>>>>4
Most of the unused content in Metal Slug X is actually beta levels from Metal Slug 3 and the entirety of Metal Slug 1's levels. I think there are one or two truly unused levels residing in X, but that's about it.
http://tcrf.net/Metal_Slug_X
I like 2 more than X since it seemed like the alien reveal was better done. Then again, if you're playing X, most probably assume you've played 2 already, so it might not be as big of a deal.
I haven't put enough time into both titles to decide on one over the other, but I have to ask a question before I forget. Did anyone else ever notice that the number on the door that goes over the character on the selection screen was never changed from "2" to "X" for Metal Slug X?