The large and small shanks are referring to the diameter of the barrel, at the threads, that screws into the receiver. The "small shank" is 1.055" and the "large shank" is 1.120". Most barrels and receivers you encounter will be small shank. The target actions, most wsm's, saum's, and rum's will be large shank.

If you're going to build it from scratch, including an aftermarket trigger, I'm not sure there's much benefit in the target action, unless you like the different port options? I have a 12F rt bolt, rt port that I'm still using the factory trigger in. If you're wanting to use the factory trigger, the target action trigger is better than the standard accutrigger.

If you're wanting an "off the shelf" aftermarket stock, there are more options avail for the standard short action stagger feed receivers than for the target actions. If you're going custom, one-off on the stock, then it doesn't matter either way.