If you are wanting one that fits both, you are gonna have to do one of two things.
But one for a 4 incher and live with the extra leather when you tote your 2.5 incher, or go with an open bottom holster, like the Galco "Silhouette" that will accomodate both with the barrel sticking out the bottom.
I have a Galco Combat Master for my 296 and it has been fine for many years. It has an open bottom. They list an L frame 3" which would likely work fine for both your guns. Here's a photo from their website. The photo calls it the speedmaster.
I use El Paso Saddlery #77 Tortilla OWB holsters for everything from 4" N-frames to 3" J-frames and everything in between. They conceal well and are very comfortable for me.