I don't understand the current fad towards shorter and shorter barrels...
But they are "handy" Perhaps they are for people who carry rifles but don't shoot them?
I would say quite the opposite... The ones likely to gravitate to "handier" rifles are the ones in the field intent on something other than paper and knowing that 2-4" won't make a bit of ballistic difference on target, but could very well mean the difference between squeezing the trigger or not.
I saw this thread start up and held back for awhile to see if the typical tone would emerge...
I will counter point the general consensus...
What follows is only my opinion... I have bobbed several dozen rifle barrels in addition to owning several dozen more factory configured short barrel rifles... I have tested barrels before and after bobbing with "same batch" loads. I have also tested incrementally on three different rifles. My observations are these.
1. Velocity loss/inch is usually exagerated
2. Handloads can often close the velocity gap pre/post bob
3. Heavier/slower cartridges (larger bore) loose less velocity/inch than lighter/faster (small bore) cartridges (on a percentage of initial velocity basis)
4. Perceived noise (volume) is not the issue that many believe, as noise is an issue regardless of barrel length
5. Balance and wieldiness are often greatly improved with 2"+ removed (purely subjective)
6. Accuracy is not affected by shortening the barrel (not subjective)
I like shorter barrel lengths for many field applications, and for specific applications, such as a truck or quad rifle. Handling and carrying on stand or in tight cover can be greatly improved... the noise in open air environments/or in range environments should receive the same care whether the barrel is 18" or 26". Some cartridges lend themselves to shorter lengths... I enjoy pistol cartridges, medium bore cartridges (8mm - .375") and straight"ish" wall cartridges (.38/55 & .45/70 etc...) with shorter barrel lengths.
O.P - where a short barrel is chosen for the sake of wieldiness, I would recommend reconciling yourself to embracing the handling characteristics and not the pure ballistic (velocity) characteristics of the given platform... ie. load heavy for caliber bullets in appropriate cartridges. It would hardly make sense to choose a .220 Swift and then Bob the barrel from 26" to 18", as one chooses the Swift for its ballistic performance. So of the two choices you provided, I would bob the .30/06 over the .300 WM.