Having done this within the past few years...
I started with an auto -- a Benelli Montefeltro. I shot very well with it, but at some point decided I needed an O/U. I ended up buying a Browning 425. I shot that one well also, but at a price. The pounding I took was taking a toll on my cheek (100 trap targets in a night would leave a golf-ball sized lump on my cheekbone), and I started to develop a flinch from being smacked so hard and often.
In a fit of anger over that, I bought a Beretta 391. I could have gone back to the Benelli, except that my wife had already laid claim to it. Of all of them, I shoot the Beretta the best. I use it for trap, sporting clays, 5-stand and pretty much anything else.
However, in the past month or two I've begun experimenting with the Browning again. This time, I'm using 7/8oz loads moving at a sedate pace. So far, the recoil is manageable and targets are breaking pretty well (but still not as well as the 391 with heavier loads). Since I reload, the convenience of an O/U cannot be overstated. With autos, you end up hunting around for your empties. With an O/U, you just pull them out and put them in your pocket. I think that's the main reason I'm willing to try again with that gun.
So, what does all this rambling mean? If I were starting over, I'd get a gas-operated auto (I'm very happy with my 391) and shoot the proverbial snot out of it. In the meantime, I would use every available opportunity to shoot other people's O/Us. See if you can find out what you like and what you don't. Proper fit seems to be even more critical with an O/U than with other platforms.
If you shoot trap (especially in a league) and get the 391, I highly recommend a T&S shell-catcher to prevent spitting hulls onto the shooter to your right. They should cost about $10, and are worth every penny. BTW, for whatever reason, I find my 391 just about perfect for trap. It patterns slightly lower than my Browning, and that seems to fit my shooting style a little better.