Hi
guess the difference comes because T5 in comparison to T8 will, at the same electric wattage, produce more light photons (efficient) and being a narrower tube diameter means it will put higher intensity light all the way till the bottom of the tank. Moreover the scalings are non-linear. i.e. as the wattage increases, T5s will out-do the T8s at a greater level.
So in your case, going from 1.1 wpg T8 to 1.6 wpg T5 will light the tank to a larger extent. So if costs are not a worry then with EI and pressurised CO2, go for T5 if faster, more growth is what you are aiming at.
-niru