Floor crunches vs. leg raises on power tower

[Anonymous]

Hello, does anyone know the difference in effectiveness between floor crunches vs. leg raises on a power tower? I'm relatively fit, a runner, but I have very little upper body strength. My husband recently bought a power tower and told me that doing leg raises on the captain chair was very effective vs my floor abs routine. With my floor routine I do 200+ crunches using my lower and upper body. At the end, my stomach is screaming for mercy. When I use the power tower, I can only do about 20 controlled knee raises at a time before my upper body begins to shake. I could do more leg raises, but I can't hold my body weight for longer. So I do 20 reps, rest, 20 reps, rest for a total of 60 reps. My abs never burn though. Should I dismiss the power tower and continue with my abs routine, or is the power tower going to be more effective because I am lifting my lower body with my core?