The 7mm Rem Mag is a great cartridge that really caught on when it was introduced, and yes, it was tops when it came to rifle sales for many years.
The big advantage is the performance with less recoil than the 300 Win Mag.
The biggest mistake I see today (and have witnessed for quite a few years actually working in the sporting goods industry), is less experienced shooters and hunters who work behind the gun counter at the local gun store, is they parrot what they hear or read online, and recommend the 300 Win Mag to many customers, including new shooters, whether they be men, women and/or youth. When asked afterwards most admit that they have neither shot or hunted anything with a 300 Win Mag, let alone a 7mm Rem Mag. Many of the new shooters/hunters end up coming back later for a "deer" rifle that won't beat them up! Even some guys do the same, when they realize that the 300 is more recoil than they want, or are willing to admit that they can handle. Not saying this is the case for all, but it happens frequently with the 300.
For years, the 175 gr bullet was very popular for hunting larger big game, as it gave great penetration with its higher sectional density. The 160/165 gr bullets were very popular for a balance of velocity (good BC), and penetration (good SD), over the heavier 175's. For those that were hunting lighter big game, the 140-150 gr bullets gave more velocity, with enough penetration, for the smaller body mass and thinner skinned game.
When I was first getting serious about hunting and handloading for rifles, and chose the 280 Rem to start out with, I read a great article about the 7mm cartridges, which stated that the 160-165 gr bullets were the optimum choice in the .284 bore for exactly the reasons I have already mentioned above.
After working, handloading and hunting a variety of big game over the past 35 year in 280 Rem, 7mm Rem Mag and 7MM STW, I can attest to the performance of these bullets for accuracy and performance on game from 15 yards out to the 475 yards, in heavy bush, mountains and open prairie, that I have taken game with in these cartridges, in these bullet weights (160 SGK's, AccuBonds, Partitions, 162 gr Interlocks, and 165 gr SGK's). The rifles have been Browning A Bolt, Rem 700 Mountain Rifle, LH Rem 700 LSS, and custom LH Sako AV, with 22 and 24" barrels. Scopes have been 3-9x40, 3.5-10x40, 3-10x42, and 4.5-14x40 (mostly Leupolds). All have produced 1/2"-1" groups with preferred factory ammunition and handloads. The Browning X Bolt is a nice rifle to carry, hunt and shoot, and those that I own and/or have shot, have all produced good accuracy. They balance well, and all that I have shot have had good triggers. (I am building my 6.5 PRC on a LH X Bolt w/ a 24" Wilson carbon fibre barrel.)
I am very happy with the 280 and 7mm Rem Mag performance, and only got into the STW because of the rifle (and have taken more game with it than any other cartridge over the past 25 years as it will shot 1" groups at 300 yards with factory 160 gr AB ammo, and 3" groups at 400 yards with factory 160 gr SGK ammon). My STW has a 24" MacLennan barrel that produces velocities (3222 fps) higher than what may be expected with this cartridge in this barrel, within safe pressures, with factory ammo, that I can only contribute to as a "fast" barrel. Otherwise I was very happy with the 7mm Rem Mag and was looking to replace the RH rifle I had with a LH rifle, when I came across the used custom Sako. It weighs 7 lbs bare, and about 8 lbs scoped. It has been a joy to carry, shoot and hunt with.
I have owned the 300 Win Mag, but did not care for it...mostly because that rifle kicked like mule. I have a 300 WSM (my first custom rifle) which shoots very well, and I have taken quite a few animals with it over the years. Much prefer it to the 300 Win Mag. Could be because of the rifle that fits me better and the felt recoil is more manageable. I also had a 338 Win Mag, and have the 338-06 and a couple 338 Federals. For those with just one rifle, the 300 (regardless of version) may make good sense...but if you have a few rifles, including a larger cartridge/caliber, the 7mm Rem Mag makes more sense as the lighter option.
Find the rifle that meets your list of needs and desires.
A scoped rifle weighing in between 7-8 lbs, that fits you, will provide provide years of quality service and enjoyment shooting, carrying and hunting with in any environment you choose to hunt, after your chosen quarry. This rifle weight, will provide more stability and steadiness when you are excited or breathing heavier from a long climb or hike, when you need to take a shot, and reduce felt recoil more, than a lighter rifle will. There is something to be said about ultra light rifles for packing in steep or rough terrain, but can be more difficult to hold steady come the moment of truth. This will be exacerbated with longer distance shooting. Things to consider as y6ou make your final choice on firearm and scope.
Having fought the thick brush and alders to get to mountain meadows and open ridges, I can say that most common barrel lengths can be a burden, and a longer barrels will be worse...this is where a pack that can carry your rifle barrel down (such as Eberlestock) comes in handy and makes this nasty travel easier. Longer barrels do have their advantages...so you need to determine what your need and desire is here, or what compromise you are willing to make.
Best of luck in your quest!