If we're talking about #9 shot, I've read that due to the small size of the pellet that the antimony levels are basically irrelevant as it can't do what it does on a pellet that small.
Can't remember the exact technical explanation but my take away was paying a premium for magnum lead #9s is a waste, save it for the 7.5s
Plus as mentioned above, the soft lead may deform a bit more than a harder lead mix, and may result in a larger pattern all else being equal.
I've been shooting a lot of 7/8oz (24gm) lately at about 1300-1350fps.
Light recoil and I get more shells out of a bag of shot.
skeet or IC for skeet.
7.5s and a LM or M choke, they work just fine for most sporting clay targets as well.
Its what I practice with.
For skeet, going to try 3/4oz next.
Seems to work fine in the 28 gauge so why not the 12?
As for the gun, I love my Beretta 390, but prefer the O/U for most my clay shooting.
30" or 32" barrels, depending on which gun I grab.