Why should I get a 14 gal versus a 1/2bbl tank? Aren't they almost the same size?

The 14 gal tanks and Half bbl tanks are similar but not the same sizes.

The 7gal and 14gal unitanks and chronicals are named after the max volume you can fit in the vessel, not the appropriate batch size. These are designed for 5.5gal and 11gal batch/brew size respectively with about 20% headspace included. 

The 1/2bbl and 1bbl+ tanks are NOT named after max volume you can fit in the tanks, but ARE named for the batch size. The Half BBL tanks are about 18gallons in total volume and the 1bbl tanks are closer to 42 gallons in volume instead of 15.5gal and 31.5gal.