What about surplus stores? There are plenty of those in San Diego.
Depending on the store, Surplus stores can be a good way to check out the boots if your nearest PX does not have them or is too far away.
Usually they will be higher than the AAFES price. On the low quarters it was nominal, but on the boots it was $70-80 difference. One local store did give a discount for appointees, bring your boot letter.
I'm seeing stores setting up Belleville & Bates displays now, and those are the ones which carry the different models like the 310 and M8
DS found the Bates low quarter suggested in the letter to be better made than some of the non-Bates alternatives. (like sold in the cadet store)
Likewise, there are construction differences between the M-8 and the 310. The 310 is a more traditional boot, though still improved over the older 390des.
The M8 (and also the Khyber) are more of a Tennis Shoe construction with molded uppers, etc. DS found noticeable comfort differences and settled on the M8, as did his squaddies.
There are also differences in the sole from a traction perspective, with the M8 and khyber being a more aggressive sole, which helps in the field.
That said, boots are a very individual thing, what works for one may not work for another. And all three are good boots.
Lastly, once past beast all the concern about specific boot models goes away as long as it's one approved for ACU wear. Last year the 2nd pair issued on R-day were contract boots not on the boot letter list. (Though much more like the Belleville 310's)
And even in beast, some companies/squad leaders may not even care, others may make a big deal. Have major blisters with an unauthorized boot and you may get more special attention than you want. And you'll also see/meet cadets who bought boots at the cadet store in beast not on the boot list.
General consensus is that getting the low quarters and breaking them in is more critical than the boots as the new boot construction does not require as much breaking. The low quarters do take some break in, being a hard soled leather shoe. And you'll spend far more time in them early on
There are many old threads on how to fit that are worth searching on.