Hysterical! moose at 800 yards? I would bet that you could "sneak up" on him a wee tad closer than that... unless you hunt him on the Bonneville Salt Flats. I hunt and therefore am obliged to kill as quickly and humanely as is possible. I would take that shot at 800 on a critter as big and tough as a moose with either round. A great shot may kill with either, but come on! You are far more likely to be tracking a wounded moose for days given where moose are found. Even with a 338 (which IMHO is the only caliber that I would personally use at that distance on game that tough) isn't really enough, unless you can hit a pie plate at that distance, cold bore, 10 out of 10 times in nasty weather, out of breath and lying on a bumble bee nest.
That being said, I go with the 7RM, hands down: more punch (retained velocity) at that distance, better BC, better SD.