Your plans for insulation sound good, but what kind of insulation are you planning on using for the floor- fiberglass? I've heard many times that fiberglass batts tend to trap moisture in the floor in our climate (we live in N.O.) Sprayed foam is the way to go- either urethane or the new "green" soy-based foam.

As to your compressor choices, I'd go with the more efficient 15 SEER unit. The variable speed air handler and two speed compressor will save wear and tear on the system and guard against those irritating "brown-outs" when the compressor kicks on. Efficiency is a great investment. You'll be glad you did when energy prices go sky high again.