That's a pretty wide open question GC. To get good answers you need to post more information, like: what are you growing in? Soil, coco, hydro? How many watts of lighting do you have? how big is your space?
Here is my personal take: I don't personally use t5's, but I've seen a lot of others use them and they work very well. Particularly as a starter light, they produce little heat and you don't have to worry too much how close they are. They produce a nice dense branching pattern if you have the blue bulbs in there, so that's a plus. In general it is better to use blue light for veging plants because red light will make the stretch more. HPS lights are red, and are normally used only for flowering. If you want to use an HID (high intensity discharge) bulb, MH ore metal halide is preferred.
You will get bigger yields from a large plant grown properly, but if your space is limited it is far better to keep the plants from overgrowing it; if not you will be dealing with light burn on top and too little light on the bottom. Its all about what space you have and using ti to the best of your ability, not about growing 6 foot plants in your closet.
-gwg
