If you hand load you are going to go through more powder per round for the 300 mag. Loaded match bullets cost more too.
You can put 8000 rounds through a 308 barrel, can shoot anything from 100gr to 175gr bullets (some shoot heavier but that is usually the upper limit for accuracy/performance coming from most experts). The SPS comes with a 1:12 twist so practically, you will be using the 168gr bullets max.
The 700 SPS tactical is a solid base, but you will have to upgrade the stock right off the bat (with bipod or shooting off sand bags in the front, the stock will flex and touch the free floated barrel, making it non-free floating during your shots). A stock upgrade is $250 at the bottom end, and can run upwards of $800.
If you get the 700P or 700PSS, you get a solid HS precision stock that won't need upgrading immediately, but you still pay $900'ish for that rifle to start.
You will probably eventually want to put a detachable magazine onto it, so you will replace the bottom metal (to accomodate AICS magazines) and that's another $300'ish or so.
Solid bipod is $100 (most use Harris). 20 MOA base and rings can be $100-$200 for the pair.
Price adds up fast, before you ever put good glass on the gun, which most serious distance shooters pay more for the glass than their gun.
Entry level, your 700 SPS is fine, but get it in 308 because it is cheaper and the barrel will last longer. Learn to shoot with that, shoot the barrel out, upgrade the barrel then. You can piece the other stuff onto it as you go and money allows.
Or spend about $1300 entry level for one of these affordably upgraded 700's:
http://www.snipercentral.com/scriflepackagedetails.phtml?packageid=2
With a few goodies you can pay a bit more for if you want.
I shoot an FN PBR in .308 (winchester pre-70 action/claw extractor) with 3 position safety that blocks the firing pin instead of the trigger seer. Comes with a detachable box magazine (proprietary and about $50 each to replace, but its a system already included in rifle package instead of the $300 for bottom metal upgrade to start. Comes with 1 or 2 piece base already mounted to action. Can get an older one with non-fluted barrel or the newer ones (PBR XP) that has flutes for weight reduction. Oh, and the stock is also a hogue overmold same as the 700 SPS, except it has a full length aluminum bedding block and aluminum rods reinforcing the stock fore end so it doesn't flex. It cost me $750.
http://www.snipercentral.com/pbr.htm
One Remington 700 to consider is the 5R mil-spec.
http://www.snipercentral.com/milspec5r.phtml
And reloading is going to cost you to get setup, especially if you buy a few splurge type items instead of bottom end stuff. Good luck finding powder right now.
God I love this hobby!