The 930 is a good gun, the 390 is a great gun.
I have both.
930:
Negatives:
Drop is only adjustable on the synthetic versions
It has a blued/anodized gas piston, Slip 2000 will strip the finish off of it
The magazine tube is blued(although very thinly) and shows wear from carbon almost immediately
The barrels are all ported
The bluing will come off if contacted by any type of scotchbrite or heavy usage of steel wool
7/8oz loads are problematic(may just be my gun, it has roughly 5000rds thru it but still has problems with 7/8oz low recoil loads and some reloads)
Pretty wood is hard to come by
Non-Chrome bore and chamber(not a big deal to me)
Poor resale value
Positives:
Gas system is easily disassembled and cleaned and will actually run a long time between cleanings
The trigger is a plus because it can easily be disassembled, cleaned and reassembled
The gun is has a fast cycle rate
You can add a magazine extension
Barrels are cheap, but ported(they can be ordered unported I have one in 28")
Cheaper but not by much(I paid $490 shipped for the 28" wood waterfowl version NIB, and paid $500, $525, $530 & $600 for 4 12ga 390 all NIB, 3 synthetic and one wood, only one Walmart version and it was $500)
390:
Negatives:
Barrels are more expensive
No Magazine extension
Trigger cannot be easily disassembled and cleaned(at least not by me)
The gas system is a little harder to clean if ran longer between cleanings, but still fairly easy to clean.
Positives:
Chrome Bore and Chamber
Better fit and finish thru out
Wood and Synthetic both use the same shims for cast and drop adjustment
Hard chrome gas system and shell lifter
Better resale value
Pretty wood seems to be readily available
This isn't a complete list, just my general observation on the ones I have. They both pattern lead and steel fairly close, I only shoot 1450fps steel loads and they pattern really well in most all guns, although some require more choke than others to get the same pattern density.
If you want either gun will work and work for a very long time, but my vote goes to the 390.
If money and time were the deciding factor I would get the 930 and have no problems with the choice. If you look and don't hurry, you can and will find the right 390 for you and I think you will be happier about the decision.
Good luck
