Wow, subjective opinions are readily available and offered like fact in this thread. Kind of like asking if you should buy a Ford or Chevy. Your opinion is probably based on your very limited experience or what your daddy told you to buy.
Yup, T5s from any fixture spill light. So do MH and LEDs. What's not mentioned here is what's spilling from T5s is probably not useful full spectrum light because the spill is from the bulbs on the outside of the fixture and chances are you aren't running all the same bulbs throughout the fixture.
PAR readings, that's another story. Yup, thousand of threads on that too, all subjective and relative to type of bulbs, age of bulbs, age of the ballasts running the bulbs, type of reflectors and whether or not there's an acrylic cover over the bulbs. And then there's relationship of the sensor to the bulb, water clarity, surface movement and the spectral sensitivity of the sensor/meter being used. Add in the experience, or lack of, by the user and the PAR readings you have read about start to become meaningless or at the very least just vast generalizations.
If you want to grow SPS it's pretty hard to have too much light. 39W bulbs are far more efficient than 24W bulbs. Larger T5s would be more efficient in every brand fixture it's just simple physics due to how a florescent bulb works and heat.
To effectively light a tank that will have coral growing up and out to the edges, as corals do, cover the tank with a fixture that doesn't rely on spill. Use a 24W if you're only going to have LPS or other less demanding corals on the sand near the edges. Growing Acros or Montis that will branch out ? Yeah that 39W fixture will make them happy and won't be blocked like the smaller fixture when the corals in the middle of the tank start to get big.
But then this is all just my opinion based on 40+ years in the hobby, subjective like the rest, however it's based on having tried it all.
