Metal Slug X is a "revised" version of this title, it includes different weapon placements, some different times of day, just in general is a different game for very subtle reasons, but enough reasons, that end up becoming a significant change.
I personally got this game for fairly cheap on a sale on one hand to complete my collection of MS games, as well as to check the differences; turns out this is the game I played as a child at the arcades and not the revised version like I thought. I personally like this version a lot more, it seems to be made with more care and I like the different times of day, the gore seems to be better implemented, and finally, the actual choose your character screen has the animations from the arcade; so when you choose Marco he squints his eyes, while Tarma gets a bugger out of his nose.
I know they're silly things to some, but generally, I prefer this version with those changes and its original soundtrack -something that also got altered in the "X" version of the game-, this version also doesn't have any slowdowns and it seems to have as many options as Metal Slug 1, X, and 3, so you can play with different filters, widescreen res, etc. Only issue I had with technical aspects, had to do with the keybinds, I had to change the player 2 keys and quit all the way into the main menu for the changes to actually take place; then I could change some of my player 1 keys the game wouldn't otherwise allow me to change because they were set for player 2's actions.