I'm looking at a 12 ga. beretta 686 for upland bird hunting, what barrel length do you like?

This is just my personal opinion.

I have 3 beretta 686. 2 in 28" and 1 in 26". Since there is not really an "action" on a o/u it is overall shorter that a regular pump or auto shotgun. The 28's seem to look more balanced and pleasing. However, if you get the 26 and utilize extended chokes then it can appear more balanced. The 28's seem to swing better and more comfortably (spelling?). I don't think the 28's pattern any better than the 26 so in the end it comes down to personal preference.
Hope this helps some.

I prefer a 26" O/U for specific upland bird hunting. I prefer a 30" O/U for specific waterfowl hunting. I prefer the 28" for general purpose, upland, waterfowl, rabbit, and clays.

Carlos' point about the overall length is well-taken. A 28" O/U is going to be somewhat shorter than a 26" pump or autoloader. Aesthetically, the thicker barrels on a 12 gauge look kinda chubby in a 26" length.

Besides, if you want something fast-handling and quick-pointing for upland game, you should get a trimmer, lighter 20 gauge O/U with 26" barrels. So go with the 28" in a 12 gauge.