Disregarding the debate over which light is better, it does seem like it really comes down to coverage.
* XR15 ... 20” x 30” coverage
* XR30 ... 20” x 40” coverage (horizontal)
* SKY ... 24” x 30” coverage (horizontal)
This is all based on SPS specs, so you can reasonably expect another 4-6” with either dimension for non-SPS.
My 72” mixed reef tank has a pair of XR30’s on the sides (2x40”=80”j and a single XR15 in the center (30”), which is about 110” worth of coverage. Everything has been spaced out such that there’s a bit of overlap between the three Radions, so there’s not really any dim areas between the lights (there is still a bit of drop-off in the corners and a slight bit along the aides and back).
The options for my tank were as follows:
* 4x XR30 (mounted vertically) ... $3,600
* 3x XR30 (mounted horizontally) ... $2,700
* 2x XR30 + 1x XR15 ... $2,250
* 4x XR15 ... $1,800
If we look at the SKY, I could probably stretch it and get away with 2 units - provided I didn’t go full SPS. But I’d really need 3 units to give me similar coverage:
* 3x SKY (mounted horizontal) ... $2,550
* 2x SKY (mounted horizontal) ... $1,700
I’m excluding the cost of any mounts or accessories, but my current Radion configuration and the cheapest Radion configuration are still cheaper than the 3x SKY lights I’d need for comparable coverage. Even if I stretched coverage with a pair of SKYs, it only saves me $100 over running 4x XR15s - and I wouldn’t necessarily have the same coverage.
I think if the SKY had a half-size version (similar to the XR15) and some decent mounting options it would warrant more consideration.