I'll preface with saying, excuse me if I appear to ramble. Just the way I compare items. Okay, going back to the OP's comments:
...a hunting rifle that's lighter than what I've got (Remington 700 sps in .308 with a bull barrel).
You are comparing apples and oranges, but asking for the apple have offer-up a nice citrus flavor.
There are two heavy-barreled SPSs: the Tactical and the Varmint. The Tactical has a short-action, as does the Varmint. The .300 Win Mag, in any rifle, will be on a long-action, and by default, will be heavier in its base compenents. Too, consider the barrel length. The Tactical has a 20" barrel, the Varmint a 24". Your .300 Win Mag will need a 24" barrel to achieve maximun results. So, now you have stepped-up from short-acton to long-action, and to a caliber that nessitates a barrel 4" longer than your current caliber. I went to Remington's website and to Weatherby's and checked weights. The following are the listed weights:
M700 Tactical SPS .308 Win: 7.5 Lbs.
M700 Varmint SPS .308 Win: 8.5 Lbs
Vanguard Synthetic .300 Win Mag: 7.5 Lbs
If I had to chose a single rifle of those listed, I would say go with the Vanguard. It comes with a 0.99" accuracy gaurantee for 3 shots. I think it is a mistake to overlook the Winchester M70 Super Shaddow. That said, they are both excellent rifles.
The weight of the rifles is but one very small consideration. When I finish adding a scope, rings, base(s), bi-pod, sling, ammo, etc, I'm already well over 12 to 13 pounds. Accessories add weight to a rifle the same as they add $$$$$ to the cost of shooting package. Heck, my Remington M700 Police in .308 Win weighs 19 pounds. I use it for varmint and deer. I think there are other areas that you can focus on to reduce your total hunting load, as well as purchase proper gear to distribute the weight so it is not so burdansome.
In closing, Vanguard.
Geno