Concerning Metal Slug...

[Che Musashi]

Ok so there is Metal Slug
Then there is Metal Slug X

Is there any difference between the two, this is only regarding Playstation of course.

[yashiro]

Series timeline:

Metal Slug
Metal Slug 2
Metal Slug X
Metal Slug 3
Metal Slug 4

X is a 'remix' of 2. (extra bits/more challenging)

So,yes, X is different from 1. It's the sequel.