Will the Garment boot be available on post later in the summer?
Fort Benning suppliers were saying June a couple of weeks ago (some still are). I'd have to assume similar for USMA.
Belleville 310 & C390 are both a bit old school. But many folks love the 390's, and some swear by the 310's for comfy Garrison stuff (milder tread).
Khybers are more aggressive sole, more of a tennis shoe fit. There are probably better options, but sadly not on the boot letter it appears. (Rocky S2V is very popular for field work, etc). It's not a bad boot, but folks seem to like the T8, 390, and 310's better. And M8's, Nikes, etc. (Nike's for rucking for time, etc).
The T8's are a bit of a hybrid, less space boot'ish than the Khybers, Nike's, etc. But a bit better for field work than the 310's and 390's. Based on Ranger School & RSLC experience these are my son's fav hot weather boot, but he had to get some Bates M8's for Coyote until the T8's come in. (S2V did not work for his foot for some reason)
The reality is, you'll be buying more boots, and have many more options available to you later. And you'll be issued a 2nd pair very similar to the 390's most likely. You'll end up trashing some, keeping others for inspection, etc.
From memory, the Khybers are a bit lighter, and the more aggressive tread will come in handy this summer. But as many have said, fit is more critical than brand/feature, etc.
I'd recommend getting fitted by one of the uniform supply ladies on post somewhere if possible. Most common mistake is to buy low quarters and boots too lose, leading to blisters. They fit more boots than anyone, and know how they fit relative to the Brannock device (foot measurer). In DS's case, he ended up taking exactly his Brannock size in Bates and Belleville. Much smaller than he thought he wore in casual shoes.
Most REI's will let you use their ramp and will help you check fit on even military boots if you ask nicely.
Main thing is there should not be lift in the heel when you walk. Nor should you slide forward if you (on carpet) scuff your sole hard forward. Never should your toes touch the front of the boot. Dodge those three things and you'll largely avoid problems.