i know i just ran into one of those charts that lets you figure out how much electric a light uses, but for the life of me. i can't figure out from where.. i did the math for a 250, and i think it was like $15/month or somewhere around that.. i would say go for the bigger of the two as the power consumption won't be all that much more, and the heat won't be much more either, and you will get a better yield in the end..
try and do a google search for how to figure out power consumption..