I did Slim in 6 a few years ago and had great results. I didn't have a lot of weight to lose at the time, but I lost several inches in my hips, abs, and thighs. My arms also got a lot firmer. Then I stepped away from it for awhile. After 6 weeks, you do get tired of the three videos, but it really wasn't too bad. For me, it was worth it.
Now, I'm currently doing the Slim Series (Debbie Seibers' follow up) set. It is more challenging and I'm finding really quick results again. It has been my saving grace during this holiday, high calorie, season.
I would go with the Slim in 6. The actual routine is very simple, there are just a lot of repititions. I don't like learning complicated steps, I just want to sweat. Lots of lunges, squats, and upper body work. An ab section at the end as well. Turn of her music and listen to whatever gets you going and the time should go by pretty quick.