The 3/8" hole is not waterproof when the antenna is removed. The NMO 3/4" is. Secondly, the smaller one can be easily knocked off. Lastly, the coax comes out of the smaller mount straight down. This makes some installations difficult, and sharply bending the coax for clearance is not a good idea. The coax on the 3/4" mount is at 90 degrees and only requires about 3/8" clearance.