The 6906 and 3913 are the same size, with the main difference for holsters being the stepped-side of the 6906 frame, making it slightly wider at that point. A leather holster will stretch a bit to accommodate for this part, but not a kydex holster.
A 6906 kydex holster will fit the 3913 but stick out a little more than necessary, while a 3913 kydex holster would probably not be good for the 6906. I do like the leather back/kydex shelled IWB holsters, like the ones from Old Faithful Holsters (who also offer them as discounted kits that you assemble on eBay).
I recently tried out Alien Gear's IWB holster, and was not very impressed with it. There are compromises made because of the universal nature of their design, and I think they churn out so many holsters that quality also suffers. Mine had poor cant and some misaligned holes, and I also ordered it for a 3913 but received a shell for a 4" 4006TSW. I won't be ordering from them again.