I have a pair (twin set) of WP-10, and a pair of RW-8. As far as flow specs go, the RW-4 is similar to the WP-10, and the RW-8 is similar to the WP-25. I like the look of the RW series much better than the WP series. The grille openings are larger on the RW's, so they're less likely to clog. I have to clean the RW's quite frequently. The RW's... not so much. The RW's are a little more expensive than the WP's, but I think they're well worth it... and I also like the fact that you can control up to 8 powerheads from the master controller (one master, up to seven slaves).