Did a little more research on the hydor pumps personally I would still get the 425 and here is why.
Price difference $1.
Hydor 240 rated for 10-15 gal
Hydor 425 rated for 15-30 gal
Also did some looking on a couple of other forums and some people were saying that the 425 was under powered for their 16gal tank but they had some corals too. My 425 in my sump is in a portion that is the equivalent to a 10 gallon tank. I have a sand bottom in the sump also. I have the pump mounted on one side blowing directly across to the other and the flow just reaches. I think if you got a 240 you would end up buying another one and be in it for $50 instead of $25. I even had it in a 10gal salt I was messing around with at one point. They are nice and small too