I am no lighting expert but just for fun i was looking at lumen maintenance (apparently a measure of a lamps lumen output over time) of MH verses T5. The T5 looked about 95% at 20,000 hours of use-
http://www.lrc.rpi.edu/programs/nlpi...s/lat5/pc9.asp
and
Metal halides lumen maintenance appear to be much lower-
http://www.lrc.rpi.edu/programs/nlpi...teristics2.asp
This is probably a nice bonus for T5 and perhaps something to consider when evaluating the Lux performace of T5 and MH on your system when at 1 year of existing use?
Although maybe I am completely wrong as i am no engineer!