Won't make any difference with the 12, if you were getting a 20 I would go 28". Either will be fast.
I have a Savage Fox Model B-D and love it. I purchased it before I joined the Navy back in 1969. It is difficult to find any SxS's in this day and age that are reasonably priced. I purchased the Stoger Uplander in 12 Gauge with 28" bbls. I bird hunt and also turkey hunt. I like the 26" length the best but I got a great deal on the 28" so I purchased it. The first thing I do is remove the butt stock and I clean all the internal parts. After I reassemble the gun I change out the factory original firing pins with stainless steel firing pins. I have had no issues with the Stoger Uplander Supreme.I am about to order a 12 gauge Uplander Supreme and am leaning toward the 26". I doubt I will shoot it much...I may hunt with it a little. It will be more for show and to fill a whole in my collection. I currently do not have a side by side and I think the Uplander Supreme provides alot of bang for the buck. I think the 26" would be a little handier to hunt with and also look a little better than the 28".