The question is not whether LED's will replace MH / T5, but when. Because within a number of years (exact number tbd) they will likely stop manufacturing the bulbs and fixtures, similar to how you can no longer purchase 60/75/100 watt incandescent bulbs or T8 fluorescent bulbs.
LED's keep increasing in efficiency, and dropping in price - similar in many ways to microprocessor improvements. Right now LED prices are dropping 50% every 3 years - while efficiency is increasing at 20%+ per year.
Once LED's cost 1/10th as much as they do today, and are twice as efficient, people will be amused that we ever had these conversations