I don't think your question has one good answer. You say "what the plants need," do you mean to stay alive, to produce a commercially viable product, or to produce maximum yield? Regardless, the answer will still be affected by the efficiency of your hydro system, as well as the variety of lettuce that you grow. Your ambient room temperature will affect it too, in that more lights might work better in winter, but cause heat problems in winter.

When you compare lights, the measure is lumens. Those lumens have to be in the correct spectrum, but if you stick with fluorescent or metal halide lights, your spectrum will be ok. I would encourage you to look at the smaller metal halide fixtures and compare their lumen output to that of the fluorescent fixtures.

There is some benefit to spreading out the light source as opposed to one big halide light in that the plants will be tilted toward the one light and you might end up with lettuce heads that are a little lop-sided looking, but if it's for your own consumption you won't care.

Sorry, typo, more lights might work better in winter, but cause heat problems in summer