I had adequate flow with the wp40 as the only powerhead, but I tend to like ALOT of flow. I cutrrently have the wo40 in my new 180g I just setup and am going to run two in there.
Based on what i saw with the wp in my 125g I would get two. The wp20 is a little less cash but its about half as strong as the 40 I believe. But keep in mind both would be totally adjustable, maybe not as precise as the vortech but you can dial it down and it has different settings for different types of flow, then you adjust the intensity.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.