I agree with Snarlingiron. Beretta introduced the 391 as a marketing strategy, not because it was an improvement over the 390. I've shot many 7/8 ounce loads out of my 390 with no problems whatsoever. I also know several shooters who shoot the 390 every week and they clean their guns about every 2 years whether it's needed or not.
The narrower forend of the 391 is only more "user friendly" if you've got small hands or if a small grip is what you prefer. Personally, I prefer the fuller forend of the 390. The larger 390 forend fits my hand better and I don't have to "scrunch" my fingers up as much to hold onto it.
Balance of either gun (or ANY gun) is strictly a personal preference issue, so I see no advantage of either gun over the other. It's all in what you like. Besides, balance can be and often is changed for a variety of reasons such as recoil reduction, adjustable cheek pieces, adjustable butt pads, etc.
There can be no dispute, however, that the 391 is considerably more complicated to clean. Also, many of the 391's had feeding problems due to a faulty carrier design. Beretta claims to have fixed the problem, yet the problem still show up frequently today. The 390's had no such problem.
By making changes every few years, Beretta can convince SOME shooters to trade their "old" guns for the "latest and greatest" new model. This is Marketing 101.......... and apparently it works.