In very general terms, an arch top (A, F, or similar style) will be louder than a flat top of bowlback. Despite that, the large-bodied flat tops are very popular in sessions for their 'Celtic" sound. There actually is no official Celtic sound though. The mandolin is a relatively new arrival on the Celtic scene. Personally, I think a lot of people prefer them for their shape, which is kind of Celtic looking, in a Lord of the Rings kind of way. (You know what I mean.).
A high quality A style arch top with F-holes will be loud. An A style archtop with an oval hole will aslo have considerable punch, and retain much of the ‘Celtic’ charm. The same is true of F styles with either hole configuration, but the A style is the more affordable and more widely accepted look.
Setting aside resonators and banjolins for the moment, much depends on the maker and, to be honest, price tag of a quality arch top. It’s not that one brand is so much better as it is that making really well-balanced, responsive, and loud instruments is expensive. You can get lucky with moderately priced mandos if you have the opportunity to try a bunch. But most folks don’t have that advantage.
The real question becomes: “How much ya wanna spend?” If I were in the market for a loud mando that didn’t cost both arms and a leg, I’d forego the Celtic look and head for an A style with F-holes. Something in the Eastman line, or if there’s enough in the budget, a lower priced Collings or Weber.
As for resonators, they can have a very sweet tone and are real sound cannons, but they lack the sound that is favored in sessions. They’re more a specialty instrument that sound great on a tune or two, but they can easily be overdone.
I’ve yet to play a banjolin that would be welcome in a session. They’re loud as a foghorn, but very harsh. Imagine the tenor banjo player with a capo at the 12th fret, banging away.