The 7mm mag and 300 Win Mag are two highly popular rounds, and I'm a fan of both. I think these two cartridges are great for everything in North America except for the big brown bears, but they can do the job if needed. Both are flat/fast shooting, both have hard recoil, and both are more expensive than a .30-06 (the benchmark). For those people who are really into ballistic coefficients, the 7mm bullets tend to have great BC numbers (I'm not one of those people). The 7mm does recoil less than the .300 Win Mag, but at the sacrifice of bullet weight. In the field it won't make much of a difference.
Having read many interviews with PHs and guides, the 7mm Mag and .300 Win Mag come up very, very often. In fact, the 7mm Mag is considered by some the perfect round for specific game (pig and sheep come to mind, the first for the extra umph for a big boar and the sheep for the flat shooting) and do great against every other game. The .300 Win Mag is the second most common backup gun for guides in Alaska, second only to the .338 Win Mag (I was shocked when I read that). Having seen the .300 Win Mag in action myself on many occasions, it really does the trick, dropping game on the dot.
For those that suggest a muzzlebreak, please don't. A limbsaver recoil pad or something of that sort gives the rifle much less recoil without disturbing shooters and guides/hunters on your flanks. I know my brother loves his BOSS on his Browning in .300 Win Mag, but I really, really can't stand shooting next to it. Do other shooters a favor and just go with a recoil pad! Please!
Both are great rounds. I own a .300 Win Mag, and would love to own a nice 7mm Mag some day. I know they are very similar, but sometimes it's not about need, but want!