Well, I own Shock 1 (MVS) Shock 2, MS1 (on CD only
), MS2, MSX, and MS3.
I'll have to agree with DolphinLord and Dion, Shock 2 isn't even CLOSE to the Metal Slug series. I was really disappointed in it, bought it only after SNK reduced the price to blow out its excess stock of the game. The backgrounds don't even come CLOSE to ANY of the Metal Slug games.
If I were you, I'd focus on the MS games, they're all better. If you're looking for MS1, just pick up an MVS version, a sac cart, and have someone convert it to home cart for you. It would be a LOT more worth it than Shock 2.