Here's my 2 cents. I am running a 4xT5 strip over my 46G plus a 2xT5 strip. I'm not sure how deep the tank is but 18 inches sounds about right.
I just happened to have the 2 bulb one laying around, and got the 4 bulb one for real cheap. Right now I have a bunch of LPS and a couple caps and a monti digi or two in the tank, but mostly LPS. EVerything is living fine and the coloration and growth on the LPS is great! The SPS is a bit to be desired in both growth and coloration.
I'm lazy and don't plan to change the lighting, but if I did I would go one of two ways:
a. Metal halide plus LED or T5 actinic
b. rip the guts of of these fixtures and buy the good reflectors for each individual bulb, and drop down to four bulbs.
I would be getting better results with four bulbs on good reflectors, than 6 bulbs on crappy ones.
But in the end......I have yet to find a T5 combination that pops my tank like MH+VHO.....unless it was a shallow tank