Played this not long after playing Metal Slug 2 to compare. To be honest, this is basically just the "definitive" version of Metal Slug 2. The only two differences I could spot were the much improved optimization (less slow downs and such) and maybe a few different enemy placements and entries that were slightly improved over 2.

That said, this is a great, classic run and gun game. Beautiful sprites and animations on some very impressive for the time hardware, and made by some of the most talented sprite artists ever.