Hi there--both are good workouts, but it depends on what you want.
I think that Cardio and Weights is not as hard as Hard Core Challenge, but it is very fun (actually I think that both are.)
Cardio and Weights has longer cardio step segments where she breaks down the combo and then repeats it 6 times (3 on each side) at the end of the segment. Then she does a compound weight segment with light weights, then a heavier weight segment focusing on the same body part. This whole cycle is repeated 4 times, with chest work tacked on at the end. The core routine at the end is very, very good.
Hard Core Challenge has shorter cardio segments--2 seem a little easier and 3 are more intense interspersed with some pretty intense weight work (some traditional, some with the band--ouchie). There are 5 cardio/weight cycles followed by an ab segment. It flows very quickly and the body parts worked are more mixed up (rather than one segment for chest, one for back, etc--I actually really like this--).
Anyway--hope this helps and that others weigh in.
Maggie