I'm in the planning stages of a similar project. I'l be building one of those two calibers on a Savage action. I chose the Savage because I can do it all myself (no machining) and the mag. box is 3.00" long vs 2.85" (I think) for the Remington.
If you handload and want to load heavier bullets you would probably be better off with the slightly shorter OAL of the 7 SAUM if you use the Remington action.
As far as velocity goes I would think you would give up 20-50 fps for every inch you shorten the barrel. Also might want to take a look at Shortmags .org
lots of good info there on both your choices.