The simple answer: Heat is for sprouting, not growing, and you warm the soil, not the leaves. Lights are for growing, not sprouting. There may be overlap, and there are exceptions, but not much that matter.

When should you start what? I have two week old toms and peppers growing for February plant-out, and i think we're cooler than Austin. That's thermally "cooler", not socially "cooler"...

You don't need a heating pad really. Room temp is fine. Such a pad will speed up germination, but won't really improve the final seedling. I am near Austin, and start my tomatoes for March 1 plant-out with no heat at the beginning of January. You don't need light until they sprout, and at that point, the more the better. I actually don't use artificial grow-lights, but have a pretty bright window, and just take the sets out in big trays on sunny days. That actually helps harden them, as well. Works fine.