Um. Metal Slug's been on for a while. I have it. It actually stars two new people in it though, but it's a pretty neat Metal Slug. It isn't as long and such as 2nd Mission, on the NGPC, but it does have graphics much more akin to the MVS look, by nature of its hardware.
It's a pretty good Slug, but still not quite the level of Slug that all the MVS versions were. It's kind of hard to pick between this and 2nd Mission as 2nd Mission had a lot more to offer in terms of at-home content. It looks pretty nice, though. Could've sounded better, but I wont fault the system for that. The cart is nowhere the kind of sizes the MVS carts have, so I'd expect the sound to be low keyed. NGPC only wowed me in the sound department because of all the work that little system did to try to produce the sound. The NGPC really was that little system that could, and they really put good effort to push that sound chip, at times.
On another note, I did find that the KOF EX2 on the GBA did really push that little sound chip, so that was kind of impressive.