Re: 7mag or 6.5/284
At that range, both will work. For me, I lean towards the heavier bullet for LR hunting. A friend is shooting the 168gr MK out of a 280 and at 1000yds, it is still hitting with authourity. Never seen what a 6.5 140gr MK would do so can't compare.
For me, I hunt with a '06 and 165gr SST bullets and would not hesitate to take a deer at 600yds under calm conditions.
Since you are hunting, I would choose the one that will work reliably in your rifle for feeding, etc. The 7rem mag is a no brainer from the factory. The 284 case sometimes needs tuning to work. I don't want any surprises when hunting. If single feeding, you may need to start flipping coins.