A few thoughts and points on regular vs short mage. Look at the factory pressure spec's, the shorts are loaded to higher pressures. Also, there've been reports that the shorts were using special powders at first that were not available to the handloading market. This is changing as evidenced by some of the new magnum rifle powders comming out. No new trick here, Remington did this for years when they bought out the 7mm rem mag. It uses 7828 powder, which was not commercially available for years, and handloaders had much trouble duplicating spec's of the factory loads.
The laws of physics still apply though, it would be interesting to see a full equal comparison between shorts, standards and longs, with the following rider:
-same pressure spec.
-same throat lead / freebore
-same weight and barrel length / contour. (this one is cute, look at how thick / thin some of these are, even in the same manufacture. This will change your recoil.)
and all of this in a common action designed to handle any one of them.
I would hazzard a guess that there ain't a helluva lot of major differences once you level the playing field.
One very handy feature in going for one of the more established players (ie: 300 win mag vs almost any other one) is that if you lose your ammo in the field, odd are real good that you can find a box in the middle of nowhere. Try to find the exotics at your local co-op or hardware store.
Last note, why all the variations? Simple. Gotta be / want / have something different and that means sell more guns. A major manufacture's rep was overheard at a large show stating that for every new caliber they put out, they will sell around 50,000 rifles for it. Gun junkies R Us! Besides, they really are just like women. Everyone has their prefferences as to what the ideal one should do / be / have, etc. It would be pretty stale out there if all the women only came in a few basic variations.
Myself, I have owned 4 300 win mags, and 3 300 short mags, 2 win, 1 rem. The meanest kicker was one 300 win short with handloads in a Savage 10. Same caliber in a 16 was not bad at all, heavier barrel in that version for some reason. The rem 300 saum was a model 7. "Magnum contour" barrel kept the recoil down and made it very nose heavy to carry, sold it quick. The real kicker, and not literally, is my custom 98 in 300 win mag, with a lightweight contour 23" barrel in a custom synthetic stock, weighing 7 3/4 lbs loaded, is actually rather decent to shoot. Better than any of the others. Guess which one I kept?