The difference between the .300 win mag and the .308 winchester is not the caliber in fact they are the same but the length of the cartridge and the powder is different thus changing the velocity. The .300 win mag has more veloctity and thus the bullet trajectory is flatter and that is what changes the long range performance. The .308, .30/06 and .300 winmag are the same caliber and have the same ranges for hunting and both knockdown animals well. If you are doing alot of target shooting I would recommend the .30/06 it has more room for powder for you to hand load better loads and get downrange performance, although the .308 is full of powder it is still accurate and the military shoots it out to 1000 yards. In fact the .308 was designed to duplicate the 06 but with newer powders and to make the actions smaller, but falls short with the heavier bullets it can't match the 06. The general rule is in bullets 150 to 165 grains the .308 excels and is the same as the 06 but at heavier bullets the cartridge doesn't have enough room for slower burning powder to propel the bullet as fast as the 06. But, it is a compact package and does it well even in the higher grains. The .30/06 is more versatile and will shoot the heavier grain bullets better due the slower burning powder you can put in it. Also, the.30/06 has more ammuntion around for ti and shoots several different bullet weights from varmits rounds to the 200 something grain bullets that give African animals a run for their money. Also the .30/06 falls only a few hundred fps below the factory .300 win mag velocity with the good handloads. For hunting the .30/06 can't be beat it has versatility and ammo is plentiful. The .300 winchester magnum has higher pressure levels and can fire a .30 cal bullet fast. This results in flatter trajectory. It is easier to shoot longer ranges with this gun and it gives it "theoretically" more knockdown power. Although shot placement is more important. The .30/06 and .308 do all of North American hunting well, I have shot elk at 400 yards and 80 yards. Moose can be taken with a .270 and have been done so by Jack O'Connor. Who was also from Arizona. He even used the .30/06 in Africa. But, if you were truly target/competition orientated and wanted a gun that would eliminate variables which one is of drop and can handle the recoil for all the practice and pay for all the rounds, then the .300 win mag is for you. Carlos Hathcock used the .300 win mag for his victories in target shooting. Generally for the average person the .30/06 is the limit at which recoil and comfortable shooting are met and anything over will be uncomfortable. I can and have shot a .30/06 all day and was fine, I don't think I would want to try that with my .300 win mag. The rifle you are suggesting is one of the best, the winchester 70 is also good. I have a Remington 700 in .30/06. Carlos Hathcock a legendary marine sniper in Vietnam, he used a Winchester 70 in .30/06 and later on Remingtons. The 700 is what the Marine and Army sniper rifles are based on nowadays, they heavily modify them though. Carlos shot his competitions and won medals with the .300 win mag. For what you ask in the question the .308 will be a shorter round and give you a shorter and somewhat lighter gun, not by much but it helps. Especially in the field although I don't mind my .30/06 some people prefer a shorter action. It will also give you good accuracy and be the best gun for hunting in rifle and its in one of the three solid .30 calibers. You might want to accesorize your 700 in .308 to shoot better for competition. All in all your rifle is a great choice and either caliber will suit your needs as most all north american big game can and have been taken with the .30/06 and many African animals as well. If you handload definately all will be a good choice. But coming from a purely store bought ammo and target competiton, I would say the .308 as the police and military using this round have caused makers to offer very good cartridges and bullets that are marketed towards competition shooting. Black Hills is a company that comes to mind. The .300 mag has been used in competition and I am sure there are cartridges out for it, but not as much as the .308. If it were my choice it would be the .30/06 with my handloads and I can give a .300 win mag or a .308 a run for its money for its the shooter that matters most. I like the flexibility of the 06 without the recoil and price. The .300mag makes the job a little easier, but kicks more and is more expensive and ammo is more expensive. If you practice like you should and know your handloads/cartridges from the store, your rifle, and the drop of the bullets from your gun you will do fine with either three. If you haven't shot heavy kicking rifles, haven't shot rifles in general, or for the average person the .308. There are too many people who don't practice because of the recoil of their rifle, and many a develop a flinch in reaction to the recoil. They don't realize there are two calibers below it that offer the same range, the same knockdown power, and a poorer trajectory but it is not that bad. People before the .300 winchester magnum were getting along with the 06 and the even worse trajectory .30-40 krag. It mostly depends on practice and skill of the shooter. Go with the .308 or .30/06 and you will definitely be happy. Be confident you have a powerful caliber and an accurate one and you can beat the average joe with his hard kicking magnum. All it does is hit the paper a bit harder and if its an animal it doesn't care what the velocity is of the bullet that hit it. All it does is go a bit flatter to the target. There is no magic bullet and all will kill fine, all will shoot paper fine, and two won't break the bank and break your shoulder especially in target shooting. It is mostly preference and if you have a .300 win magnum and practice and have it for the benefits then that is fine too. Good luck! (I have one too for the benefits of 1000 yds shooting in competitions but have used the .308 exceptionally well) Hopefully that helps you sort things out a bit, its hard to give you a definite answer, now if you said .223 vs .50 that would have been a bit easier. But .308 vs .300 mag, its a shorter gun, less recoil vs flatter trajectory but harder kick. That is what it boils down to. Edited To Add: Sorry Bound posted while I was writing, I was hoping he would show up. He summed it up great! It is true the .300 win mag will shoot better with winds or be better for you to figure out the trajectory and is great for 1000yds. The .30/06 does have the airpocket issue and is technically not as accurate as the .308. And the .30/06 has been disappearing from the line, I still remember when there were a fair number of them on the line, one of them being me. I threw the .30/06 in there for a versatile hunting round and for informal target shooting plus minor competition. So if you are doing target shooting for competition reasons the .308 would be good with the .300 win mag the best especially 1000yds.