Hey - I was in the same situation as you about two months ago - I was doing workouts with Cathe daily on FitTV and was thinking about buying the DVDs but wasn't sure it would make that much difference!
Here's one thing I want to add too ... when you do the FitTV, even if you fast forward through the ad breaks, you still get that thirty second break ... on the DVD you just keep going and I found that that combined with the full workout in its original entirety really made it more fun!
Off FitTV I was doing Step Blast, Rhythmis Step, SJP, KPC, Lo Max and Imax 2.
Here's what I bought back in the beginning of March:
Rhythmic Step / Interval Max / Maximum Intensity Cardio
Step Blast / Step, Jump, Pump
Imax 2 / Cardio & Weights
Imax 3
Lo Max
Kick Max
That's the order I would get them again too. Rhythmic Step+ is probably the most intense out of the group because of MIC, and Interval Max is hard too.
Hope this helps!