Pay a few $$ and do a proper load calculation so you get the proper size and it will save you a lot of $$. Try this one:Load Calculator
In your climate a higher SEER will save you a LOT of $$ in the long run so I would buy a 15 SEER. You have lots of humidity and I would seriously look at a 2 stage unit. They run longer and give you much better dehumidifying which allows you to keep the tstat higher and still feel comfortable. I deal Lennox as well as other brands and would recommend a ECM motor variable speed air handler and a 2 stage unit. If you pay $1000-2000 more for better equipment and amortize it over 10-20 yrs it really is not that huge a cost for better comfort. You get what you pay for.