I have probably blown more egg money on these two categories of cartridges than any others and have been using both since the late 70's IIRC.I've been loading for and hunting with both since then, too.
Most of my rifles were chambered for the 300 Win Mag,300 H&H,and 300 Weatherby;(these three are separated by 50-200 fps,depending on barrel length,throating, how hot you load them, etc);in the 7 mag category I've mostly used the 7RM,the 7mm Dakota,and the 7mmWSM.
Back in the 80's,I had it all figured out....the 300's were "better";used heavier bullets at the same velocity the 7RM gave lighter bullets, the paper ballistics showed the 300's to be more powerful,and I used the 300's up close and to quite long range to flatten elk and other stuff.The 300's are splendid cartridges, among the best as general purpose BG cartridges IMHO.I always felt that with the heavier bullets they really are more of a "medium" bore, like a 338 IMHO.
But the extra "advantage" of the 300's came in the form of heavier rifles,more recoil,and greater powder consumption to move bullets of roughly equal ballistic properties to the 7mm's at the same velocity.This, of course, means they kick more,and I agree with Crow Hunter on the issue of recoil thresholds.I have watched some pretty bad 300 mag shooting by some guys who were better served by a 7x57.It always struck me that the 300's were a bit over the top for a lot of people and are best reserved for seasoned, hard core riflemen,and not for those burning a box or two of cartridges per year. This type should avoid a 300 mag like the plague IMO.
I've had numerous factory and custom rifles for all these calibers and found that I could build a 7RM(Dakota,WSM,etc)to be lighter and still be easy to shoot.A comparable 300 mag had to be heavier;I tried building a 7.5 pound 300 Win Mag (once)and it turned out to be pretty unsatisfactory.It kicked like 3 mules.
Creeping doubt entered my mind on an elk hunt in the 80's where I watched a little gal weighing 110 pounds soaked in oil, flatten a big herd bull in mid-bugle at about 500 yards with a 7RM and a 160 Partition.Since then I used the combo on elk myself,and seen it used many other times on a variety of other stuff,and IMHO,the game ends up quite as dead from the 7's as it does from the 30's,especially if you stick with well constructed 160 to 175 gr bullets in the 7mm's.
I've got a 160 gr 7mm Bitterroot here that traveled the length of an elks neck from behind and stopped under his chin; it smashed vertebrae and tough neck muscle all the way,still weighs over 155 grains and is expanded to over 60 caliber.The wound channel was a complete train wreck, fully the equal of anything I've seen from a big 30,including the 200 gr Nosler Partition, which I have driven from forward of the hips of a big bull clear to the off-side shoulder,breaking it.
I have a hard time drawing a fine line which makes the 300's a more suitable class of cartridges for elk sized stuff,as I firmly believe a properly loaded big 7 is fully capable of taking on large soft skinned game,and I would not hesitate to take one against any game available here.
Looking back on all this,if I had it to do all over again,one rifle for all mountain and open country hunting of trophy soft-skinned game from pronghorn all the way up,I'd likely do what Page did decades ago and get a 7 Mashburn ,Dakota,or 7mag of similar capacity,load a great 160-175 gr bullet,and shoot everything.(For those rare circumstances where this is not "enough", my second rifle would be a light 375H&H).
I know I could have a lighter rifle that recoils less,kills stuff about as well,and is generally more easily managed.With todays great bullets, the situation is even better.JMHO and YMMV
Sorry for being so "long"!
