I've owned 7 rem mag's since 1974 and STW's since the early nineties.
Both are tremendous calibers and will do anything you want out to 800yds.
However with the STW you can keep going for another 200-300yds on just about any game in N. America and push any bullet 150-200fps faster with it as well.
As for barrel life if you load either to the max and shoot the hell out of it you probably won't get much over 500-700 rounds out of them before the groups start to open up.
However for 99% of hunters they won't open enough to be outside of the margin of shooter error for you to even notice until you get beyond 1,000 rnds.
Don't shoot max weight bullets at chargest that exceed recommended maximums and don't get it hot and you can shoot them a long time.
As for the 7mm Rum, I don't see the sense in it. For the additional cost in recoil, muzzle blast, and powder to me it's just not worth it.
For big game rifles I currently own 7mm STW, 300WM, and 300 Rum. I have all the gun I'll ever need for anything with them.
If I could justify the need for owning anything bigger I'd go all the way and build a .375 Cheytac and skip everything it and what I currently own.
Back to your question... .
If you build an STW, so as to get the most out of it get a 26 or 28" Barrel.
If you build the 7mag, go with at least the 24" but I'd say go with the 26".
With the same length barrels for both calibers the STW will rarely give you more than 100-125fps more, but you have so much more powder capacity it really shines with a couple of extra inches of room to burn that powder and build velocity before you let it fly.
As for carrying in the field, 2" more barrel really doesn't make much difference.
