Read up on light penetration with T5s before you make that your decision. See the stickies in the lighting section. Also keep in mind that the plants will double in size (roughly) once they go into flower. I would go with a ceramic metal halide (which fires in an HPS magnetic ballast). Your grow area is pretty small so you could get away with 250w although a 400w would rock the box and leave room for expansion. Whatever route you go you have to consider heat extraction.

Point of clarification (for those who remember Roberts Rules of Order...): You can veg a clone as long as you like. It won't impact the end result. The fact that the clone is the same age as the mother demonstrates this. You can keep a mother plant for years and still get buds from flowered clones that are as good as the first run of the mother. Otherwise clones would become less potent the older the mother gets. But that is not the case. That said, a clone will tend to branch more than a seedling, which may or may not be a good thing.