Depends on what you mean by "reach out". Do you have regular access to a range that's more than 800 yards? If you do, will you regularly shoot at that range or more? If so, you have a solid argument for the .300 Win Mag.
The .308 can be pressed out to 1000, but will go subsonic before it gets there, affecting accuracy somewhat. I went with the .308 because I know it's fine out to 600-800 and will still give a good hit probability on a terrorist sized target at 1000.
I have heard the barrel life of the .300 Win Mag is dramatically shorter than a similar .308, due to the higher pressures and velocities. Perhaps a .300WM shooter would like to confirm or deny this rumor?
I second the comments on not skimping on optics and will add don't skimp on your base and rings either.