I belong to several fitness forums. Here are my faves:
www.videofitness.com and Firm Belivers/ Firm Ya Yas (you'll need to google it because for some reason it is being blocked here).
I have read countless of posts that members have said that Slim in 6 is beginners to intermediate and folks gets tired of using it. The Firm has been around for ages and they range from beginners to advanced and they have consistently given great results.
Of course, you can always do a rotation where you change your routine every few weeks or so. Please feel free to check out those websites because there are reviews, great people, and suggested rotations, too.
GL!