Re: 7mm/08 AI vs 7mm RUM
I can't be sure from your post if you're satisfied with deer at 600 yds or if you also plan to harvest deer at 1000 yds.
At 600yds either cartridge will be fine. At 1000yds the 7mm RUM is the only choice, if limited to these two options. You won't be able to maintain bullet expansion velocity at 1000 yds with the 7mm/08 AI. For that reason, I wouldn't even consider using it at 1000yds for any large game animal. Varmints and targets - yeah.