It all depends on how you used it. BUT I would probably go with a 150 and that alone. They are more readily available and cheaper than the 70w Halides.
You could do a pendant, which IMO look the best.
You could get a micro chiller which are not too terribly expensive. They mount on the side of your tank.
You could build your canopy as an open style with the top slatted instead of solid and the back open.
You could install some fans into the canopy, but this would really accelerate the evaporation, and I dont think that this would be the best idea for such a small tank.
I am in the planning stages of a 7 gallon nano with a 150w halide as we speak. I just have to find where I put all my money