hi bro,
a MH bulb can last atleast 1 yr before it looses its intensity....but for t5, most reefers replace them after 6 mths of firing and also that depends on the number of hrs running.
well, diy MH setup are definately cheaper than 6-8 t5s. btw, the cost of 80watts t5 are expensive...so do the tubes...
if i were u, i run 2x250watts with 2 t5 suppliment..... or maybe 3x250watts to have a good spread...