I marched the last 6 years with the Blue Stars. I can assure you, this is not the case. Yes, some people move to different corps (you'll have that even in the top 6), but the feeling of family you get while marching with BS makes most people stay. I'll admidt, when I went there in '02, I was planning on using BS as a stepping stone. By mid-season, I realized how much I loved the corps and by finals, there was no way I'd ever leave.
I also have first-hand experience with the Colts organization and know that they have a large family-atmosphere as well. Both corps would be great choices. I have friends that started marching with me in Colt Cadets in '98 and stayed until they aged out last year.
I know you've already corrected your "lesser-known" statement, so I'm not yelling at you, I just want you to look at the past few years and notice that both corps are growing and moving up in the scores. But congratulations, you seem like a person who wants to march because it's drum corps and a great experience, not so you can get a ring (of which Blue Stars have gotten 4 times, although not in Div I: '89, 93', '01, and '03--i've got mine that year)
Feel free to PM me or AIM at flag4321. I always love to talk drum corps and I can answer any questions you have about Blue Stars and can answer a lot about Colts or find the answers via my close friends who march/marched there.