I'm always totally in the minority on this but what the hell. Slug X is my favorite Metal Slug game. I remember when it came out SNK kind of hyped it up as a "fixed" Slug 2. I was at University of Florida at the time in college, and I remember playing a lot of Metal Slug 2 in the arcade on Campus. I preordered Slug X from NCS (COD by the way, anyone remember those days? A UPS guy knocked on my door one day and I had a money order for like $293 ready for him). Anyhow, the game came and I really got a very clear view of what SNK was trying to do. To me it just played better all around, it was so playable from start to finish even with 2 player simul. The slowdown was a million times better.
And…. I don't know if this is the case with Slug 2 or not.
But there is a TON of awesome debug stuff in Slug X if you poke around long enough!