Dolphin Blue plays more like Contra than Metal Slug. I once owned it, but I really didn't like it, the slowdown is ridiculous, even worse than in Metal Slug 2, and it gets really memorizing-heavy later on to avoid cheap deaths. I haven't played the AW version of Slug 6, so I can't really comment on it, but the PS2 version goes on forever. In a boring and chore-like sense.
I'd go for Dolphin Blue of the two though.