I've no experience with the 7mmSAUM so take my opinion despite it's limitations.
Of the two, I'd choose the 7mm Rem Mag all day, every day and twice on Sunday.
They are similar enough ballistically with the edge going to the 7mm Rem Mag. If I'm wrong here please correct me!
However, the 7mm SAUM can use a shorter receiver making for a shorter, lighter nicer handling rifle all else being equal.
I just read a similar thread here in comparing the 7mm Rem and the 7mm Weatherby Mags. What was interesting was the discussion revolved not so much about the slightly greater velocity potential of the Weatherby over the Rem Mag.
The discussion became mostly about ammo and/or reloading components. In this regard, the 7mm Rem Mag has a clear advantage in availability.
However, a good point was made that since the 7mm Weatherby is not as popular there may be a greater potential to source ammo/components for it as it is "looked over", a less popular chambering.
Otherwise, the 7mm Rem Mag brass is available in vastly greater quantity and quality as well as at better prices. Powders, primers and bullets are all similar, but brass availability is tantamount and the 7mm SAUM isn't nearly as popular as the 7mm Rem Mag.