The MB amp is a 6 ch. Do you have plans for the other two channels?
If not, I would definitely take the savings and go with the JBL.
Speakers -- At that price difference you'd have to think the JL's would be quite a bit better, but TWICE as good? That seems unlikely to me. So I would lean JBL on speakers as well. Although, I'm a cheapskate and wouldn't blame you at all for going with the JL's. That 2nd set of JL's would be getting into the extreme zone for me. A boat just isn't a place I'd be willing to spend a bunch of money on equipment geared toward SQ when that's the furthest thing from an SQ environment. Durability, respectable SQ, ability to get reasonably loud -- would be (is) my focus for marine audio.
HU looks fine. Nothing great spec-wise, but typical perfectly adequate.