It all comes down to dead spots. You'd be minimizing them by running one on each end of the tank, as opposed to only one larger pump. Having them in the back of the tank would probably leave more dead spots than having them on the sides.
Maybe one of each. I had a wp40 in my 150 48x30x24. Gave me good movement in one whole half. But flow didn't make it to the other side. . I had a peninsula BTw. and my aquascape was pretty much mountain in the middle with almost a 360 view. If you just have a back all one may be enough for flow
I went with a wp25 on a 4ft tank & if turn it all the way up i get a hole on one side. If i turn it down I dont get enough flow to the other side, i would do 2.
I run 2 wp25's on my 5' tank. There on either side of the center overflow facing away from each other. Its more than enough flow I get water movement all over the place.
I have two wp40's on a 5' wide 30" tall tank. The overflow is on the left corner with one powerhead attached to it facing the opposite end. At the other end and forward a bit is the other powerhead aimed the opposite direction creating a circular current. They are both controlled by the JBWave controller.
I can do many different configs from pulse to streaming. These wp40's easily can tear it up or just calmly flow the water. Im very pleased with the lack of dead spots. I highly recommend them.
Just go with the 40s and run them on the low setting. If you ever want more flow, or go into more flow requiring corals, you already have the pumps, and will not have to buy additional/new ones.