question on XM bulbs???

[oohmiller]

I have heard that the 20K 250w XM bulbs put out a better par rate than the 15K 250w XM bulbs. Does anyone know how true this is? I switched from the 15ks to the 20ks and am just wondering if I made a good choice. I am running these on a standard 120g tank (I think 48x20x20) for 10hrs a day with actinics for total of 12, on 1 hour before, off 1 hour after. Suggestions on if this is enough or if anyone thinks I should leave them on longer or not, I know some leave their halides on for up to 12 hours a day.

Thanks
-Andrew-