Personally, I like T5s for a bunch of reasons:
1) They work. They have a long history of doing the job and doing it well.
2) You can change the color after you buy the fixture. While LEDs are somewhat adjustable, T5s have a huge number of options in terms of bulbs and can be changed to fit the needs of the system or the owner's color preference.
3) They don't produce shadows. A lot of LED owners have started adding T5s because of the very sharp/deep shadowing the LEDs cause.
4) They are fairly inexpensive to start. A used fixture or a retrofit kit and you're in business. I made a fixture using a simple wooden box and retrofit kits, hung it over my 40B and love the look.
I admit to being somewhat biased after my DIY LEDs killed 50+ coral colonies. My fault, I admit that, but they did great under T5s for years and it only took 1 month under LEDs to kill them all.
Good Luck,