depends on your route... theres probably nothing better than an MH for veg. For a 15 square foot grow space you'll probably want a 400 or 600watt... 600 would be best, 400 might be a tad on the low side...

i think 400watt MH puts out around 35,000 lumens, so that'd be just under 2500 lumens per sq foot which is about the minimum you'd want. assuming you';d then switch to a HPS for flower, a 400w HPS would put out around 55,000 lumens or 3600 lumens per sq. foot which is kind of... ok... but not ideal

i would do 600watt if its in your budget and your grow space is tall enough to keep the light at a safe distance.

if you are dealing with a shorter grow space, you might want to go the CFL / Flouro route...

if you are going the CFL route, you'll want a mix of 2700K and 6500K colour temperature bulbs (they say on the packaging somewhere most of the time). You'd want to go heavy on the 6500K bulbs for veg, then heavy on the 2700K bulbs for flower.

Flouros are more or less the same story as CFLs... you want to look for "T5" high output grow bulbs