If its over a 6' 120g then you should probably go with at least 4 units of 60. A lot will depend which emitters you use. Very high output/high efficiency CREEs will be more expensive but you need fewer.
I use 10w emitters and have 28 over a roughly 6' 165g mainly LPS reef with some sps. If I was going full SPS reef, I would probably add another 6 or 8 emitters. What I have now works very well for my application.
In your case, if you went with 10w emitters in sets of 6 per heat sink (i.e. 60w per heat sink/per driver), then you would have one every foot & a half or so. If you went with 5, then you would have them one every foot with 6" from each end.
If you're using 3w emitters, I would probably use more emitters, more heatsinks, but maybe not the full 60w per heatsink. My 10w have 110 degree spread so I get good coverage per emitter. 3w emitters will give you a different spread/coverage.
For instance, my EShine 60w Cree unit placed very close to the surface does not fully cover my RSM130 and if I raise it, I don't expect it to be intense enough at the bottom. These have 90 degree lenses. Each unit is roughly 16" x 6" wide. So I would need to use at least 4 or 5 over a 6' 120g IMO.
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