Both have its pros and cons...
I have used both..
T5,
Pros, Initial setup is cheap and DIY is relatively easy and with the different color temperature you are able to come up with combination that you are going for, the PAR is enough and proven to be able to color up corals and promote corals.
Cons, Heats is quite high if you are using a lot of tube, Used 10 tubes for my cube tank last time and it is hot!!! Maintenance cost is high as you require to change the tube every 6 - 9 month if not when the spectrum shift, you will have algae problem. High running cost.
LED,
Pros, Heat dissipate from the heatsink and not from the bulb itself. Bulb life is very very long. Low running cost compared to other lighting.
Cons, Initial setup is quite high, DIY is difficult if you have no electronic background, Usage of the LED is too short to prove its ability to sustain corals and also if the bulb will maintain its PAR and spectrum without 5 years.
This is what I come up with..