The two uppers are totally different from one another, the Watson's is old technology compared to the BOHICA.
The Watson's upper has to be shotgunned to unscrew the bolt to place a round on it. Then you have to screw the bolt back in the lower, put the upper back in place and slide the receiver pin back in place. The BOHICA is set up like a traditional bolt action rifle, no need for any tools to load it.
The thing you have to ask yourself is, how long do you want to wait. The wait now for a BOHICA is about 1 year unless you can find someone else selling theirs. There are other uppers on the market like the Safety Harbor, Ferret50, ALS (long wait also).
As for the 24" barrel, you'll lose velocity, probably about 25-30 FPS per inch. The only thing you'll lose is really long range shooting ability as the longer barrel will let you reach out further.