Very good guns. I've owned a 390 and currently have a couple of 391s and in a couple of respects the 390 is a better gun. It has a simpler mechanism and is easier to clean and a metal trigger guard as opposed to the plastic on the 391.
The downsides of the 390 are its slightly heavier weight, wider forend, handling that doesn't match the 391 IMO and it does not function as well as the 391 with very light target loads.
A 390 will have the older Mobile choke system and a heavier barrel. This isn't a concern as there are lots of Mobile chokes still in circulation and older 391s use the same system.
On the used market the 390 holds its value well. Parts shouldn't be an issue as the gun is still made in the U.S. under Beretta licence as the 3901. Prophet River has brought them in and as I recall they were around the $1,000 mark.