Fair enough. If efficiency dictates all (and you're right looking at longevity and par to my tank, that it will)
To be clear, again, when I say "efficiency" I mean "how much light hits a coral per watt." It's important to be very clear because sometimes words like "efficiency" get thrown around with very different meanings.
If you're considering efficiency by that definition, the problem becomes very easy to solve. Identify the spectrum you want, choose LEDs that help you get something close to that, then buy the ones that produce the highest amount of usable light per watt used. Manufacturers make this data readily available so it's pretty straightforward. I'm not here to be a brand-fanboy, but in the spectra typically used on fish tanks, Cree products pretty much always win this comparison.
To give you an idea, I pulled data off a LED vendor's website. This vendor sells Brudgelux and Cree products. They list a neutral white Brudgelux LED that produces about 80 lm/w at 700mA. A neutral white Cree XM-L produces about 127 lm/w at 700mA. That's a 58% improvement in output in terms of lumens/watt. Of course the XM-L costs a lot more, which leads us to the next question...
If you bring in upfront build cost, the only real complication is, "what's the expected ROI?" If you spend 20% more upfront, will it pay itself off in a year, or ten years? This is a little harder because it depends on more variables (what you're paying for the LEDs and drivers, what your electricity cost is, and so on).
I've seen many cree builds running at 50% power, so I'm assuming I can put them 4' from water with 40-60˚ optics similar to:
Santoki's build or use a wider spread although this will depreciate par, I plan to keep sps at the top/middle of the tank.
I would only caution you to be very careful when looking at existing builds - be sure you understand all the factors. Drive current, optics, height, coverage, bins of LEDs used, and so on.
the second question to answer is; are the 1st generation bridgelux so inefficient that 3 times the leds wouldn't put out the same amount of light closer to the water, with better coverage.
Don't tie height above the water and coverage too tightly. With any common brand of LED, you're going to have a range of optics to choose from, basically allowing you to pick height and coverage independently of the fixture's overall output. Design for a specific output, then adjust height and optic choice to get a balance between coverage and intensity.
Additionally at what point do the two builds become equally priced (180 3w old tech inefficient power consumption vs. cost of buying 150 Cree (130x18/ 16))
Again, it depends on where you shop. There are a few respected vendors selling "real" Cree LEDs at prices so close to what Bridgelux LEDs typically sell for that, to me at least, it makes no sense to go with the cheaper LEDs - unless you're only planning on keeping the fixture for 2 or 3 years, which probably isn't the case given the size of your tank.