I've been using the 315's for a year. Normally grow about 7-9 strains and don't go for yield, more variety, quality, pheno hunts, seeds and the like. Growing for yield and efficiency is about genetics and filling your maximum sq. footage with bud. I can run all the Blue Pyramid I want and won't get shit for yields in comparison to something like a Blue Dream or a cross I have that will pull ~1 oz. per gallon of medium. I did one run with 4x Blue Dream's in ~3.5 x 3.5'and pulled 13 zips total, one plant got some kind of root issue and pulled only 2 zips. Pretty sure with either of my producers, a clean 3.5 x 3.5 x 3.5' run it would pull an elbow. Not bad for a 315w bulb, SCROG/Hydro would likely pull more. Is it better than a 600? Too many variables unless you did a proper side by side, same environment, clones from same strain etc.. Does it matter? imo yes for flexibility and coverage, if I can run 2x 315w and pull 1.5 - 2 lbs. going for yield, I wouldn't switch to a single 600, or even the double 630 CMH. 2x 315's with the hood (Sun Systems) can cover 3.5 x 7ft (24.5 sq. ft.) well and in that setup I'd bet it could pull more than either 600 or 630 all other factors being equal.
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Cheers