A lot and neither is terribly efficient. Be prepared to throw a lot of power at things. IDK if Fi makes 21"
I did notice the other day while looking for parts that Pierce Audio has neo motors and they have some extra-large cones. Might be worth reaching out to them. I've seen their subs in action around here and they're pretty solid and I've had good luck with their softparts in the past.
Check them out and they are very impressive indeed good power handling lots of features and lightweight worth the money in my opinion, look very solid.