Metal Slug X is an example of what game companies had to do before online patches were a thing. When going over Metal Slug 2, which you can read about here, I found the game to be a chuggy sluggy mess, grinding to a halt whenever the player dared to breathe. X is the marker pen crossing out such issues, and adding zombie dogs as it does so. I guess sometimes you have to give just a little bit more when saying sorry.
X has the same stage count, the same weapons, the same playable characters, the same pretty much everything save for a few minor additions, (like those zombie dogs) such as were items and enemies may pop up whilst charging through the stages. If 2 didn’t suffer from technical issues, X would be a rather pointless ‘remix’. but of course those technical issues do exist, and so X must exist to. Slowdown still comes on over to say hi, but it says hi when screens are filled with missiles, enemies and swaying bridges, not because you took a leap into the air surrounded by nothing. It’s on par with the original Slug, but the additions to the sequel make for a more varied solider/mummy/alien blaster. As it should have been in the first place.
If you are after more Metal Slug, choose X not 2, and then wonder why on earth Hamster would bother releasing both when X is clearly a way to right all the wrongs from the past. Money.