Couple of thoughts. If by a not so perfect shot you mean hit it in a bad place, not so sure a 6.5mm bullet from a 6.5/284 or a 7mm bullet from a 7mm Rem Mag will make any difference in recovery, with bullets of similar construction and SD. If you mean the ability to take a less than a perfect shot angle, say slightly quartering to or away, it will come down to bullet construction and the SD of the bullet in most cases with either. I'd recommend staying pretty much with broadside shots with either round, with the possible exception of 175 gr. or heavier bullets in the 7mag.
Looked at another way, with the same bullet construction, there is nothing I'd sensibly do with a 7mm Mag 160gr. bullet that I could not adequately do with a 140 gr. bullet from a 6.5x284. Where the 7mm Rem Mag, and 280 to a lesser degree, separate from the 6.5x284, in my opinion, is with 175gr. and heavier bullets. Yes, you can use 160's in the 6.5x284, but we are running into lower velocities due to powder capacity issues. For you, if you want to Moose hunt, probably your 300 mag would be the choice, and the other rifle for the rest. Given that, and not hand loading, 7mm Rem Mag would be the one I'd pick.
If bullet construction and impact velocity are roughly the same, a 130 gr. 6.5mm bullet will penetrate about the same as a 150 gr. bullet in 7mm. Both have SD's of about .266. A 140 6.5mm is just slightly superior to a 160 7mm in SD, but very close. Play with this calculator:
http://www.beartoothbullets.com/rescources/calculators/php/density.htm?bw=140&bd=.264 Generally speaking, if all else is equal, the bullet with the higher SD would be expected to penetrate better.
If you are OK with more expensive semi-custom to custom ammo, either the 6.5 or 280 would work as well. My 280 with a 24" tube, averages 3,013 fps with Barnes 145gr, LRX's. It also loves 175gr. Nosler partitions at just over 2,700 fps. Between the two bullets I'm good for jackrabbits to Moose. Both loads are not hand loaded, but semi-custom/custom loads. If you decide on the 6.5x284, make sure to study cartridge OAL as it related to the action and magazine box of the rifle you are interested in. A lot of the higher velocity offerings may come with a longer OAL and not function in all magazines/rifles.
Above all, have fun and be safe!
