I have a Rizzini Omnium 20g and a Miroku Mk70. The Omnium has the same internals as the Artemis, but with basic wood and engraving. I have also put a few shots through a Beretta "EELL" 20g and an AL391 semi, sorry for being vague but I don't know much about Beretta's lineup.
In Rizzini's favour:
I'm a heavily built 6'2" and find the Rizzini to be better fit than any Beretta I've shot. Berettas always seem a bit "low" for me, and I end up looking down on the sighting rail. My little Rizzini 20g fits me better than even 12g Berettas. I find the balance of the Rizzini to be superb -just- beting the Berettas
For the money, IMO Rizzinis have better wood and engraving.
In Beretta's Favour:
Fit and finish. Wood-metal fit on the Rizzini is very tight, but un-finished wood can be seen where the stock meets the action and stands slightly proud. Being a furniture restorer, this drives me mad. Also, there's a few spots of loose solder on the external barrel "seams" (sorry don't know the technical term). However the internal quality of the Rizzini seems perfectly good and the gun seems very solid and built to a tight tolerance. In comparison the fit and finish on both the Beretta EELL and AL391 was faultless.
In conclusion:
Berettas and Rizzinis fit quite differently, so go for the best fit. If you like the fit of both it's a toss-up between the aesthetic qualities of the Rizzini and the finish of a Beretta. Both are excellent handling guns, and really I don't think you could go wrong with either.
Hope this helps.