No difference except the so-called self cleaning piston, which was more of a
marketing gimmick than of any real value. The Optima barrel is an overbored
barrel, and doesn't have an extended forcing cone.
Word of advice: Retail price of the gun is very high, and there frequently are a lot of used 391's in very good condition for sale on this forum for not much more than half the price of a new one.
I own a Urika Trap Optima which is a back up gun for me and a Urika 2 Sporting Optima which I use for skeet, 5-stand and sporting. I can only highly recommend one if this is what you want, or think you may want. But, I really can't say that one piston is better than the other. A Beretta will go a long time between cleanings. And when it's time to clean you do what you have to do. Just have the right tools to do the job.