What about rebuilding the original seats with more foam and/or gel inserts. Motorcyclists seem pleased with gel seats, though I've never tried one.
http://www.myseatdoctor.com/index.html
My experience with air ride seats (4 million miles) is that they work great for violent road surfaces, like the George Washington Bridge in NYC, but day in and day out the quality of the foam in the seat makes all the difference.
http://www.myseatdoctor.com/index.html
My experience with air ride seats (4 million miles) is that they work great for violent road surfaces, like the George Washington Bridge in NYC, but day in and day out the quality of the foam in the seat makes all the difference.