I have a question. Would the exact same person be a "better" officer going one direction or the other? In other words, would they be in better physical shape, would they be more ethical, would they be "smarter" (academically speaking), better long term leaders, etc.
Of course their will be trade-offs.
Is the academy population self-selecting? It's a reasonable question.
Keep in mind the Coast Guard doesn't have an ROTC program, so I have to compare CGA to Coast Guard OCS.
I'm not going to try to argue which program has better people. I know of at least one (but I think I know of two) people who went to CGA, were disenrolled, and eventually attended OCS. They were "good enough" for CGA, but they were good enough for OCS. She isn't the most impressive officer, but then, not all CGA-produced officers were either.
I can also think of at least one instance of an officer candidate having some kind of "honor violation" that, as a cadet, would have forced him out, but as an OC... he lived to see another day. That sent ripples through the Corps of Cadets.
And I know from talking to OCS grads, especially about the Coast Guard Academy Alumni Association's publication "The Bulletin" that OCs often feel like second class citizens. That makes sense when we think about the Bulletin, because the Bulletin isn't for OCs... its for CGA grads.
The first ship I was stationed on had a Citadel-produced ex-Navy officer, turned CG enlisted, turned OCS grad commanding officer (an O-5 eventually promoted to O-6). The XO was also an OCS grad, and also prior enlisted. The OPS was an OCS grad, no prior experience, and the EO was a mustang (I think direct commission). Five of the JOs were academy, and the other three or four were OCS. The CGA grads didn't make much of the academy upbringing (except for the fact that we all knew eachother, either as classmates or adjacent classes) and the senior members in the wardroom, the CO and XO, as well as the EO didn't bring it up either. The OPS on the other hand (remember, the guy with no prior military experience), always had something to say about his time at the Academy (Coast Guard OCS occupies a small wing of Chase Hall, where the CGA cadets live). Eventually, after a number of little jabs, he was asked to produce his class ring... and after that toned it down a bit. Generally, when OCs made fun of cadets from "Canoe U." or "the Factory" they were asked how Summer Camp in New London was.
My experience as a cadet, and then as an officer, was Coast Guard Academy generated officers tended to down-play where they came from with other members of the Coast Guard, unless it was a number of academy grads... then they might tell some stories.
Once I left my cutter and went to CGHQ in D.C., I started to wear my ring again.
I agree that an Academy education prepares you better for that initial tour, because you've been living and breathing "military" for four years. That said, we tended to be less developed socially (we thought all the girls loved us and our manly uniforms....) and our fashion senses were hurting...
But after the inital shock to new members of the military, I would guess comissioning sources in some measures, even out. Academy grads have a better network, both within their academy and with other ring-knockers, and they have a good looking start for their resume. After that it's all about performance, academy, OCS, direct commission or ROTC...