I've had a 20x23' excavation done next to my house for an exterior
carport. The hole has been filled with 6" of compacted 3/4" road-fill,
ready to have a concrete slab poured on it.

The concrete contractors that have bothered to call me back (2 out of
9!) have given me wildly different estimates, and slightly different
specs for the job. The estimates I'm going to settle by trying to get
one or two more, but I wanted to ask about the specs.

The slab is butted against the house foundation, 23' deep, 20' wide.
The expensive contractor spec'd a 6" thick slab, #4 rebar plus wire
mesh. The cheaper estimate said "rebar plus mesh" but didn't spec a
thickness, just 4000 PSI. Given how many yards he said I'd need
(6-6.5), he seems to be pouring a 4" thick slab. Is that sufficient in
northern New England? He will put an expansion joint in the middle.

I don't know a lot about concrete structures, and want it to last. What
kind of questions should I be asking?

I also got some quotes for asphalt paving, just as a backup. Any
particular reason to like one over the other? I tend to prefer concrete
slightly for the durability.