I've been using LED grow lights of various kinds for over 5 years now, and they are revolutionizing artificial light for plants. One reason for this is that LEDs can focus specifically on the colours of light the plants need, and not waste power on other colours.
Colour temperature (measured in Kelvin) is less relevant to the discussion in the case of grow LEDs, because it applies mostly to hues of white light, akin to the sun. But photosynthesis doesn't use the whole spectrum (at least, not equally). It uses mostly red light, and some blue, and not much if any of the other colours, although I don't doubt that there's still some debate about the perfect combination, and that may also vary from one type of plant to the next. So, most LED grow lights appear red when you turn them on, and the vast majority use both red and blue LEDs, usually at a ratio of about 4 or 5 red for each blue. A light for vegetative growth typically has a higher ratio of blue than a flowering light, but it will still be mostly red. You can buy pure red and pure blue lamps, but they're intended to be used as supplementary lighting. Some companies are now putting a small amount of infra red, ultraviolet, or white into the mix, but I don't know if there's any science to justify it or whether it's just a gimmick.
What I've found is that lights with fewer, higher-powered LEDs are better than lights with more, lower-powered LEDs. That is, one 3W LED will serve you better than three 1W LEDs. This is especially true when it comes to lights made with individual LEDs that are less than 1W each, which, although they "work," seem to have relatively poor penetration.
I've never tried an LED replacement bulb in a T5 fixture, but I'd start by comparing the price of 6 LED bulbs with that of a new fixture. I've had pretty good luck ordering the stuff from China through eBay, as it all seems to come from there anyway, and there's very little available locally.