No you can do both or should do both, but you feel it more when your a stright. At least that's what I feel along with quite a few others. And since it's been a while since I did KPC, I wasn't sure if that was the one with the ball or not, and I didn't have the that DVD accessible at the time, so I just wrote up the crunch that you'll feel the most, in the lower abs. Depending on how your bulit you'll either feel that being stright is a harder crunch or being slanted like Cathe is a harder crunch. But for the reports I get back is it seems if your stright you feel it the most in your lower abs no matter which crunch you think is the hardest. Once you feel those lower abs start to really work and stretch, you can take it about anywhere, the floor or even on your step. Which can really give you a hard ab workout, I set up my step with downward angle, with three risers under one end, and only one riser under the other end, I put my head where it closer to the floor, put my butt where it's higher in the air and crunch up, and then do obliques, but for reverse curls I'll turn around so my head is the highest, and butt the lowest, I grab a hold of the step above my head and then bring my knees up. It gives and awesome ab workout, but I honestly didn't want to give you a ton of things and kind of turn you off, and think it wasn't worth reading threw everything I wrote. As I love to help people but at the same time I don't want to bore the poor person to tears. But do experiment around with floor work, step work, ball work, working your abs, as long as you got proper form and really tightening those abs each time you start a movement and keep them tight all the way through until the end, you should be perfectly fine and doing them correctly. The main thing is keeping them stretch and tight, no matter what your doing to them. It's just like a blanket on a bed, you can stretch out the top part, but if you don't do the same with the bottom part there is a poof in the middle of the bed.
I hope this helps and sorry about being confusing, I just had forgotten what all KPC had for an ab workout. But as long as you follow Cathe and keep in proper form and and your abs tight no matter what, you should have nice flat abs in no time.
Kit