RE: 30/06 vs 300 Win Mag
I'd have toside with those who wonder why you'd want to rebarrel your .270 into a .25-06, but to each his own.
Considering that you'll already have either a .25-06, or in the event you want to keep the .270, and as much a fan of the .30-06 thatI am, I'd probably suggest you go with the .300 Win Mag. Like someone else has already posted, any time I see .30-06 ammo on the dealer's shelf, I generally see .300 Win Mag there as well. I can't say the same for the "other .300s" though(Ultras, ShortMags, Weatherbys, etc.). But, I can't say that I see all that much .25-06 ammo anymore, unlike .270 which is everywhere.
Your .25-06 (or .270)will easily cover deer-sized game, and you're not gaining much but bullet weight by going to the .30-06. Truth be known, if all I had were a .25-06 or a .270, I'd still go elk hunting with it. I don't think you NEED a .300 to down an elk, but seems you don't have as much in between as you would if your deer rifle were a .243 and you were looking for a .30 for your elk rifle. In that case, I'd lean more toward the .30-06.
If you were to have only ONE rifle, I'd recommend the .30-06.