i have t5s on my 90g, i am switching to a 180 and it is way cheaper to buy a 2x250 halide setup than it is to upgrade my t5 setup to cover the 6' tank. so the cost isnt an issue anymore for t5s, more of a preference i would say, also add up the cost of replacing say 8-12 or more t5s to cover a 180 compared to replacing a few MH bulbs and the cost difference is nill. check out my little red house for growth shots of T5 lighting and check my sig for my current t5 setup. i am not saying that t5s suck because i really like them and they work awesome, its just that it is not pratical to go t5s, fo me that is, over my new tank
Tim