A good day to you too, ED.
I'm remembering a few of the criteria, that I was looking for in a panel. I went with smaller 100w units, and now have 3 of them...they're not quite equal to a 600w hps.
LEDs/Drivers. I wanted USA made (you may find similar Japan/EU high quality chips), like Cree or Bridgelux. A well engineered diode, is crucial to both light output, and longevity. The drivers are also a concern, but less of one. They are easily replaceable anyway.
Spectrum/Output. I wanted a system that put out much of it's energy as PAR at canopy levels, and in the correct ratios of color. My units have 11-bands, but I don't think it's imperative, that one have that many. I went for as much output as I could get, in the widest band, at the plant, for the money. This is where the rubber meets the road - the engineering of the unit. If it's a too-good-to-be-true company, the unit might just be an amalgam of parts - and it will often show in the specs. Of course, the specs can be weighted etc...so, on the next one part
Reviews/Warranty. I wanted to see legitimate grows with it on the web, but also I wanted to hear the enthusiasm in the voice of the happy customer, leaving reviews on other sites. I also wanted a true, no-questions-asked type return policy - for a minimum of 3 years. I used mine once, already. The power went out one day, and my light would not light up on one side. Got a new light, no problem.
I hope you find that helpful. If you find a link that you are interested in, you can leave it here, or PM it to me, anytime.
The zucchini are for tomorrow
now I need some cheese - such great problems to have!