I agree with most. Look at both Pedro's and Ruby's and make your decision. I have stayed in an air conditioned room at Pedro's and stayed many times at Rubys. Heck, maybe you can stay at Pedro's then maybe you'll meet someone who can split with you at Ruby's.
Take just a day trip to Caye Caulker. In my opinion, there's not enough to do there for 4 weeks.
I can't speak though about the ruins and mainland... sadly.