I've never personally shot a 300 mag, but have been around folks that do shoot them. I wouldn't go near it for a primary rifle.

Yeah, it has a lot of energy and can be very flat shooting for long distances. But, they kick like mules and are kind of overkill for whitetail. I don't think you will be very comfortable shooting it much.

One of the best things about a 270 is you can throw a 130gr round in it that will still knock a deer on its @$$ and you can shoot a lot without bruising your shoulder. It is also pretty flat/fast with that lighter round. The 30-06 is gonna have a little more energy but is also pretty comfortable to shoot with its lighter rounds.

I would only go for a 300 mag if you expect to be shooting more mule deer than whitetail, with elk or something large thrown in too.

A 300 is one of my favorites

Cabelas has a good sale going on right on several different brands of rifles. Just got a flyer in the mail.

Depends on area you are hunting, what range do expect to be shooting?
I hunt east Texas, I have a stand in heavy woods along a creek and use a Marlin 45-70, great on hogs, excellent knock down power, no tracking in the thorn bushes required. At 60 yards and in its proven great for multiple harvest of deer and hogs.
I also have a stand in some open pines and use a Marlin 308 where I tend to have longer range shots.

308 Winchester ammo is pretty easy to find right now, 45-70 as well. 308 Marlin, can be found on line right now but I warn you it can disappear from the shelves and web sites. I only use for hunting so a stock of ME 308 is easy for me to keep on hand.

I love my 45-70, for more versatility a 308 is great choice.