While I've had a smoldering interest for years (and have played TONNES with semi autos and pumps) I just started seriously checking out the over under world this past year. I've had beretta silver pigeons in 28 gauge, 20 gauge, and 12 gauge, a beretta 692, a citori 725 12 gauge, an older ruger red label, a couple of Webley and Scott 2000s, and now a cynergy sporting model.
Out of everything I've tried, the cynergy fits me the best: it's the most comfortable for me and I hit best with it, but I had to rotate through a tonne of difference over unders to figure it out. I think the best advice I'd have would be to echo others...unless you don't mind the hassle (and serious investment of $$) of trying everything on your own dime, seek out others to try as many different types/makes as you can before you make your choice.
Cheers,
Brobee