We have a lot of them over several different tanks, some sitting on the tanks, others hanging. The longest I've had one set up was a 30" fugeray over a 12 gallon long (36" long). I had it hanging roughly 8-10" from the top of the tank and the spread was fine. Lit the whole tank to a low light level. I had moss, riccia, floaters, and anubias all growing well. No CO2. Another one we've had running for awhile is a 16" Ray 2 hanging about 6" above the the top of an 18" cube. We always have floaters so this decreases the light intensity and made it perfect low light. Without floaters I'm guessing it would be medium. No CO2 on that one either. Also have two 2.5g tanks with 10" fugerays sitting directly on the glass top. I need to dose excel and keep snails in them or my glass and driftwood gets GDA. I could keep going, but you get the idea. There is a lot you can do with these lights. If you want to hang it fairly high I would say go with the Ray 2. If it's going to be close to the tank, just not on it, go with the Fugeray.
ETA: Here's a link to the PAR data for the various models: http://www.plantedtank.net/forums/sh...d.php?t=189944