Yes, I think the Terminator workouts are a lot harder than Cardio and Weights, and all three are circuit workouts. The only thing I do not like about the Terminator DVD is there is no ab work.
For other circuit workouts, I personally do Boot Camp and High Step Challenge more than Cardio and Weights because I think they are both harder. Bootcamp does not have a lot of Cardio in it, but it is intense and I love how my core feels afterwards. If you use the weights Cathe suggests in Bootcamp, the weight portion is great, too. Most of the High Step Challenge cardio is not intense, but the last few cycles are pretty tough, and the weight work is thorough, and it's a newer workout and a lot of fun. My absolute favorite circuit workouts are probably on the Hardcore Extreme DVD--that DVD has 3 compilations of workouts from the Hardcore series, and they are super tough! I love all three of them.
Finally, there is circuit max. I don't use it a lot (it's older, and although the cardio is real good I don't think the weight portion is as hard as some of the other workouts mentioned above)--that being said, it is a favorite with a lot of people on this forum.
Good luck making your decision!!!
Dukechick