With 1" drains (you always measure at the smallest point), you can drain 600 gph per drain maximum, so your pump shouldn't be any stronger than 1200 gph actual flow (i.e. real flow after factoring in head resistance). In a normal setup where the pump just returns the water straight up to the tank, factor about 5 feet of head. Also, if you reduce the size of the return pipe from what a given pump is setup for you will further reduce flow. With a resistance of 6'-9' of head, you will get about 1200-1700 gph flow from a Reeflo Snapper. The Dart would flow even faster (which you cannot utilize with two 1" drains).
Simply put, you need a pump that in actual use (head factored in) pumps about 1200 gph of water since that's all the water you can drain to the sump per hour. You can valve any pump down on the return side of the pump (just put a ball valve in the return line). So, it makes no sense to spend more $ for gph you can't use!
By the way, the "made for ____ gallon tanks" recommendation is almost useless unless they know exactly what you are using it for. Most of the Reeflo recommendations are regarding closed loop designs where the pump serves no purpose in filtration -- it just provides flow. One large drain (2" usually) goes straight to the pump, and the water is returned right back to the tank. That is of use in reef systems where tidal action and extremely high water flow is needed. A freshwater tank doesn't have the same requirement of flow, and you don't have to be anywhere near 10x per hour for biological filtration (some would tell you the slower the better for bio).
