There is one manufacturer, Evergrow in China, producing both the D and IT series and the Reef Breeders Value and Photon series respectively. But Reef Breeders has an exclusive deal with Evergrow that they build their lights to different specs. Also Evergrow will not sell you a Photon 48", you can only get this unit from Reef Breeders.
Ultimately, the biggest benefit from buying from Reef Breeders is the 2 year US warranty. Which in my opinion is an absolute must. 4 months into my Photon 48 the controller started going nuts and switching to random intensities, fortunately I caught it with only minor damage. But it was refreshing to find that a simple e-mail and 3 days shipping, I had a new controller and didn't have to come out of pocket. Had this been purchased on eBay or directly from Evergrow I would have had to wait a month to get it fixed and they probably would of demanded the return of the faulty controller at my expense.
Also, because these lights are put together in simplicity you can fix anything on them yourself. Another great feature.
On the Reef Breeders website they recommend a Photon 24 or 32 or 2 value fixture over a 40 breeder. I guess it depends if you plan to keep anything that requires good light on the ends of the tank. If you go with a unit smaller than your tank, perhaps you could put 120 degree optics over the LEDs on the ends of the fixture.
But personally I wouldn't put a 16" fixture over a 36" tank.