Is Sonny 2 a version of Sonny with several improvments, or is it a completely new game? I need to know if I should complete Sonny first or if I'm just wasting my time and I'll go through all the levels I've been through before in Sonny 2?
It's a continuation of the Sonny storyline (different maps, different enemies) with the same style of combat, but with different classes. If you're in it for the storyline, then you might as well play the first one up to the crazy bosses at the end, so you know what's happening. The skills are mostly new with a few carryovers, so there isn't much to from that.
It is a sequel. Storyline wise, you'd be best off completing the original first. They are both very fun games in their own way though, so you won't exactly be wasting time.