I don't know that suspension is really necessary to smooth out your ride. You can get a sprung saddle (I ride a Brooks Flyer and love it) and also use fat slick tires like Schwalbe BIG APPLEs or Fat Franks. Most of all, you need to find a bicycle with geometry that agrees with you and your back. I can tell you that my old british roadster bike with large fat tires smooths out the front end bumps much better than my mountain bike with front suspension fork was ever able to. I don't like riding suspension bikes on roads since they absorb a lot of your pedaling effort and have extra unneeded weight. I sold my mountain bike long ago.