1. Reef LED versus fish-only LED---how to buy. What to consider.
Fish only, most any led will do fine, even the Current and Marineland with 0.5 watt leds
Mixed reef, any fixture with 3 watt leds, even if only driven to 1.9 watts like the Mars Aqua
SPS reef, most fixtures with 3 watt leds. Coverage is more important (below).
2. Cost advantage? or is it a wash, if you were absolutely starting from no-lights?
Some leds are more expensive than others, but IMHO, most of the 3 watt inexpensive leds are good enough to keep sps tanks (mine has been running for 3 years now and is over grown with sps and lps corals). You save on bulbs and that's a good thing. The fixture itself is a bit more efficient than t5 and even more than MH. If you were to pick MH, would you need a chiller? I did. And my home A/C ran more as well. So I got less than $1000 on leds and saved more than the cost of them in under 2 years (t5 bulbs, MH bulbs, chiller electrical use and home A/C electrical use).
3. What is par?
Photosyntheticly Active Radiation. It's the light wavelengths that plants (algae) use to do photosynthesis. And the zooxanthellae (algae) in our corals use mostly (but not exclusively) blue spectrum. Which is why most led fixtures have lots of blue leds.
4. How does width and depth of tank affect the brand you buy and the number of units you need?
Width is most important as it pertains to coverage, just like how long should your t5 bulbs be or how many MH bulbs do you need. As for depth, shallow tanks of 12" or less can use most any led (assuming it's dimmable and these days they all should be). 12" to 30" deep, most any 3 watt led can create more than enough PAR to 30". Deeper than 30" and you need to pay close attention and even do some testing on your own. But there aren't too many tanks deeper than 30", sure there are some, but is a small minority.
5. What about pre-set programs? Doing your own?
You don't NEED programs. Your fish and corals don't NEED programs. This is an option that is strictly a human desire. I like having programmable dimming so I can have my tanks feel a bit more natural. I like having sunrise and sunset so my take looks different as the day goes by. Heck, the wave maker pumps at the ends of my tank alternate in 6 hour cycles plus 2 hours of overlap where both run just to simulate tides as well But my fish and corals could care less about the lights. The tides do tend to keep the tank a bit more stirred up and cleaned out.
6. What about building your own?
If you are big into DIY, go for it. It use to be the cheaper way to go just a couple of years ago. But with the proliferation of inexpensive led fixtures from China, you can get very good quality (even programmable) led fixtures for less that the cost of a similar DIY fixture.
7. Advice for running LED in your tank: percentage of power, etc. Acclimation to light. Special care when lights come on after a long period of darkness, say, a power outage lasting more than a day.
If your system is new and you are using leds from the start, run them at half power and ramp them up over time. I recommend that most 2 channel fixtures run at 50% blue and 50% white to start. Then if you up the blue to get a cooler white look, dial the white down a bit as well, say 60% blue and 40% white. Over time you can increase the total percentage of power. I run mine with a 5 hour sunrise (all blue the first couple of hours at dawn. Then a 4 hour midday of 100% blue and 50% white (I've taken 3 years to get them that high). Then a 6 hour sunset (the last 3 hours are all blue). I figure I get 4 hours of good PAR during midday and an additional 1 or 2 hours before and after midday. So that's 6 to 8 hours of enough PAR for the zooxanthellae to do photosynthesis. BTW, most zooxanthellae will only do photosynthesis for 6 to 8 hours and then shut down any way.
I have no concerns about going back to my normal power levels after a 2 or 3 day power outage or enforced darkness to eliminate algae or bacteria blooms. The pigment protections that corals develop to block light they don't want doesn't fade away that fast. Just like a human with a good tan, it doesn't fade away in just a day or even three.
