I first want to say that my advice is only a suggestion and not definitive.  I have learned that some peoples ideas of flow don't agree with mine.  Growing up near the cost and near rivers I know natural bodies of water have a tremendous amount of flow energy, the few little powerheads most hobbyist use are not equivalent to this.  I also don't use or believe in DSB's so I have no personal experience with this latest trend.  I am old fashioned and generally avoid fads- 2" of aragonite sand of 2 to 3mm generally is what I use. 
The ideal is to have one pump on each side.  I have a 4ft 120gallon aquarium and I use 2 7400/2 Turbelle on a 7094 multicontroller.  This has suited me quite well and I attach a photo.  My return from the sump adds about 500gph to this 2100gph of flow giving me a 20+ turnover rate.  My friend Bruce Kelley uses 2 2002 and a 6080 Stream on his 125- 6ft aquarium.  The 2002 are pulsing throught the use of a 7082 controller.  The 6080 creates a static length wise flow.  Bruce always uses a plenum.  Return flow is about 500gph.  His total flow is about 4000gph.  He keeps almost exclusively SPS though.  He noted some damage to a Montipora capricornus after first adding the Stream.  Realisticaly caps grow near the surface in seas that at the roughest in storms would pulverize a human being against the rocks so I can't imagine that it is too much flow and the corals did fully adapt- just wanted to through that in because a few people have said the pumps are too much and damaged their coral- only because they are used to poor flow.  My friend Aaaron Strong uses 2 6100's on a 7094 controller in a 180gallon 6ft reef tank.  He uses a DSB and also has patches of mircle mud.  His tank is a real show stopper of almost all SPS and a very natural patch work of LPS, polyps, mushrooms, and clams.   At first the 1 6100 he started with was to much and stirred the sand but as the sand bed matured this stopped and he added a second one.  
My advice is to try to find a hobbyist with some Tunze pumps that you can visit personally.  Barring this I would probably suggest 2 7400/2 or 2 6000 Streams on a Multicontroller 7094.  The Streams do require patience- we are about 4-6weeks behind.