The main difference that I see is with the smaller 7mm caliber, recoil is less. With a 30 caliber, recoil is going to be more, simply because of the heavier bullet. Ballistics are virtually identical if using comparable bullets.
7mm has a better BC. 
Well.. when comparing HEAVY 7mm bullets to HEAVY 30 caliber bullets the BC race is pretty close!
Especially in the case of using Berger Bullets..
If you compare a ballistics chart of a 7mm Rem Mag shooting a 168 Grain Berger shot at 3050 FPS vs a 300 RUM shooting a 210 grain Berger shot at 3050 FPS... the velocity at 1000 yards is almost identical. What is different though is the ENERGY at 1000 yards... the 30 cal from a 300 RUM has over 300 lbs more lbs of energy.
Personally, I have NEVER shot a 300 RUM or 7mm Rem Mag that were not almost identical in recoil.. so I vote for the 300 RUM.
But hell.. at the end of the day both are awesome. So.. pick a favorite I guess
