I do high school gymnastics and club gymnastics. I am training xcel platinum. On top of that I dance and am in chorale and show choir which both require time outside of school for rehearsals. If she really wants to do both teams, I think there is a way to manage both. I know level 9's and 10's who compete USAG and high school and they're very successful in both.
I also think the practice time might be beneficial, but it all depends on how strong the program is. I don't think she will lose skills, especially if she is still going to club practices. I know that for my state's high school gymnastics program, if a girl is to miss a high school meet for a USAG meet, the high school team is disqualified. This could be different in your area but if you plan accordingly you still can go to USAG meets often. Although you may be attending two meets in one day or weekend.
I really like doing high school gymnastics. I think that the atmosphere at meets is a lot less stressful. Of course everyone wants to win and get good scores. High school gymnasts can be just as serious as other gymnasts. However, high school meets show more sportsmanship. Every team cheers for both their teammates and girls from other schools. At USAG meets, I feel it's so quiet and less friendly. That is just my opinion of course.
Whatever you & your daughter decide to do, I'm sure she will enjoy it. Best of luck!