Actually I found none of them useful. What am I after? Saving up ram memory by swiping to close apps or something? iOS do that work for me. Access the multitasking bar? What for?
Zephyr sound better to me if I have to chose, but again, I access the multitasking bar once in a week, and I'm not in trying to swipe left right from one app to another.
It could be great to have it on iPad though, but not on an iPhone IMO.