While I've never really discussed this with individuals who were prior enlisted, one of the biggest Mental challenge "Differences" I've concluded is the TYPE of leadership. Some say it's easier; some say more difficult.
In enlisted BMT, your average age is 17-19, but your MTI's are TRULY ADULTS. Many are E-5 - E-7's who are usually in their late 20's to 30's. You're use to taking "orders" from adults. Parents, teachers, coaches, etc. At the academy, while the age of the incoming cadet is also 17-19, the cadre running BCT, and pretty much the academy as a whole, is about the same age. Maybe 1-2 years older than you are.
For some, this can be difficult to accept. For others, the whole place stresses them no matter who the person yelling at them is. So asking what the difference is, only really means anything if you have already been to BMT as an enlisted airman. In that case, the mental side of BCT training and having someone probably younger than you, trying to exert their authority and trying to train you to do something you already know, is probably the most challenging. Hopefully, being most prior enlisted go to the prep school first, their able to get this mental attitude adjustment taken care of with a much smaller class size and older instructors at the prep school. Then, when they come to the hill, their able to "Play along" better. That's one reason I think the prep school is such a good thing for prior enlisted. Even if they've been taking a lot of college classes and are academically prepared to jump right into academy academics. They have to get their mind readjusted to accept the role of basic trainee again instead of looking at the cadre and in the back of their mind saying: "Shut up KID, I've already been here".