The t5's are a larger bulb with more surface area. So your answer is it's all about how you use the lighting. If I had to pick one over the other I'd say t5 is better but if I'm spending that kinda money. I'm going mh hps or hid. But if it's a veg plan only. Like a mother keeper you might have a better plant with the t5's and still save cash on the power bill
thanks for your post, it will be for a veg / mother only setup, and you nailed the theory behind the ?, it has everything to do with the heat generated over a MH, but mainly the power bill.