all you need flowing through the sump is enough gph to feed whatever you have in there- a skimmer, a refugium, reactors...all of them don't require a ton of flow.
IMO, 1200 gph through a 15 gallon sump is overkill, and you may be left with a micro bubble problem.
i would go with the 700 gph model for 2 reasons:
1. it is plenty for your system
2. it's half the size of the 1500 gph model, and your tank is small enough to notice the extra space.