I attended both last year. Both are good for information and such to get a small taste of the different academies and their respective cadets which is the point of the sessions, but like John mentioned, USAFA's SS is much more laid back. For me, it was almost more like your typical summer camp with the cadets referring to us as "campers" at some points and just general not being on the same level of interest of what was going on in the SS as I saw in the USMA cadets while at SLS. They actually told us by the time they had gotten to our session, they had given up on teaching us how to try to march or get into formation because it wasn't worth trying as we wouldn't do well with them. This kind of attitude sort of killed some of the enthusiasm of the week for me.
Part of that is because as I picked up from my cadre as USAFA's SS, this was not her first or second choice to be spending her summer, but she was doing it as required for leadership requirements (which I totally can understand). The cadre at USAFA also were rising firsties for the most part while USMA cadets were yearlings so I don't know if the age difference would have an impact as well.The USMA cadets impressed me a lot more while I was there in their interest and willingness to push you and open up to you while you were there instead of just acting like they were stuck babysitting you. The cool thing with the USAFA cadets though was that since they were older, they had already chosen branches (? not sure if that's the right terminology) and you could do a lot of talking ot them about that while the USMA cadets hadn't really reached that point in their careers yet.
Please do not take that as me hating on USAFA cadets; I have the utmost respect for the cadets that I was able to spend time with while I was there for SS and when I was out there for a recruiting visit, but the quality of presentation of the different seminars, I found USMA's much better and inspiring. At USMA, everything was about being a part of a team while my element at USAFA did not have the same experience. Doolie day was definitly a good experience though that you wouldn't get at USMA's SLS.
Really, you have to take both experiences with a grain of salt anyways as they're only able to ever give you a taste of what academy life can be like to see if you can imagine yourself doing something like that for four years of your life as well as providing a forum for you to get any and all questions answered. If you leave one of the academy seminars without asking all your questions about that particular academy, shame on you. Look to take advantage of the opportunities you can. Any of the seminars are really only what you make out of them