There's not a lot separating the lower Prospex from the higher 5's.

The only things I can think of are the Prospex models will always have at least 200m water resistance and all the other things that make them ISO rated divers, whereas the Seiko 5 dive watches are usually 100m. The other thing is that the Seiko 5 will always have the day but the Prospex doesn't necessarily have to.