Mutant Mudds actually feels very rewarding to clock, but if you can only pick one, go with VVVVVV... I haven't played it but I say that because it's on sale whereas Mudds is not, and I've heard very good things about it. It's a shame it isn't available here, or I would give more input.
On the subject of bosses, I agree that I don't see how they would work since the main challenge is the obstacles in each level. It just feels so good when you finally beat Secret 5-3, for example, with the useless Power Shot, and I don't think they could parallel that with bosses... I'm interested in seeing how they'll work in the sequel.
(still loved it, though)