In the case of 72" tanks its just seems so much simpler and cheaper to go with halides than worrying about 12 36" T5's which is pretty much the standard 125g set up although its not written in stone you have to use 12. the initial cost of halides is going to be cheaper. for a12 bulb 36" TEK retro kit (six 36"x2 kits) its going to cost over $800 with bulbs. Ice cap is going to cost around $1000 (three 36"x4 kits) with bulbs. almost any brand of 250w SE retro kit with standard batwing reflector (ice cap, sunlight supply, coralvue, ARO) is in the $220 range ($660 total) with bulbs. come replacement time you only have to change 3 bulbs instead of 12 but cost will be about the same (12 $20 T5s will cost $240, 3 $80 halides will cost $240) so while its not cheaper its easier and simpler. most halides last a good year. T5s may last about 6 months longer.
As far as your power bill with T5s your still talking 468 watts so that would be like running two 250w halides instead of three. you can equalize that as well by either a) upgrading to one of the more exotic reflectors (luminarc, lumen bright, lumen max. make sure it has lumen lol) designed to cover a 3ft area and use just two halides. your only concern here is the center bracing or b) go with 175w halides which will cut back watts from 750w to 525w, virtually the same as the T5 and I know for a fact 175w halides are intense enough to keep anything in a 125g. both those options will save money in the long run and intially for the 175w set up (around $600 for thee 175w kits. you wont save any money initially upgrading reflectors and using two halides since upgrading reflectors arent cheap).both those options cut back on heat and electricity but dont quite match T5. close enough so it shouldn't matter though. I'd personally (and did) just run three 175w halides. I actually only ran two but my tank had no center brace.
if you want to add actinics skip past the T5 HO and just get two 72" VHO with UVL super actinics and call it a day. thats about as simple as it gets and for actinic supplementation it still doesn't get any better. if you dont think so go look up the best tanks running halides and see what they use for supplementation. T5s equivalent (four 36" bulbs) will cost about the same so thats really up to you.
