If a comfoertable seat is what you want in a car then get one with a comfortable seat, I didnt get a peugeot because they allegedly have uncomfortable seats. My citroen C5,my merc. sprinter and my vauhall omega have excellent seats, having a fat bum probably helps.
French cars generally have the softest seats, almost like sofas! Peugeot, Citroen and Renault. If a new car is not an option, you can get seat covers which have extra padding which will make the seat more comfortable. :-)
Maybe your suspension is very stiff too and therfore you are getting more bumps than normal - what car do you drive at the moment? You can get lumber cushions that you can use safely in cars that may help - your local halfords will probably have a good range of options.