Re: 120 or 140 gr in 7mm-08
If you're considering long range hunting, then you'd ideally want a bullet that will provide the necessary terminal performance at impact velocity.
Your twist may dictate the longest/heaviest/highest BC bullet you can expect to stabilize reasonably well. And, your cartridge may limit MV.
Hence, you can compare ballistics curves relative to factory ammo or estimated MV and determine which bullet appears to provide optimum range/performance for your rifle.
Magnums will usually perform well on paper with the heavy bullets. And, non-magnums will often prefer the medium weight bullets.
Lighter FB bullets often provide best accuracy at shorter ranges and as they reach their mid-range targets faster, they're often preferred for varmints that may be on the move and don't often afford a lot of time for dialing trajectory.
Because Barnes bullets are essentially all copper, they're lighter than lead jacketed bullets of the same shape/size. Hence, you can typically figure on stepping down one weight class.
In the end, theory goes out the window and it hinges on you working up a good load for your rifle and having the dope and confidence to make the shot.