I picked up a used 4', 8 bulb TEK unit cheap. Put brand new ballasts in it, new ATI bulbs... Very cost effective method for lighting my 60"x30" aquarium. I was looking at several grand worth of LED's... Ended up spending under $300. IMHO, it looks great. Better than the new ATI unit... why on earth they don't offer those lights in black is beyond me.
Bought a 2 bulb 24" Odyssea T5 for my macro algae refugeum off of Amazon. Cheap, grows macro algae just fine, with the included 6500k bulbs.
The main advantage of the ATI units is that they are actively cooled. Most inexpensive units are not. If the unit is going to be hanging in the open, active cooling probably isn't really needed. If, on the other hand, you're going to enclose your T5 unit in a canopy of some sort, then some kind of active cooling is probably called for.
At a higher price point, you can get dimmable T5's, but I've never seen the need. I turn on 4 bulbs, then the other 4 an hour later, and reverse the cycle at the end of the day. A simple timer (or RKL, in my case) is all you really need.