If you are doing it just for the lighting, then I recommend getting the low wattage Moon bulb, because if you get the higher wattage one and you have to put it on a dimmer switch because of the added temp, then the light gets almost invisible.

40W(IF) You are only doing to illuminate the tank, but the thing is with these the bulb is taller then the regular size bulb so you have to have a deep dish dome light.

75W(IF) You are doing it to illuminate + a little heat(not much).

I would recommend one of the higher watts if you're doing it for the temperature, because with the 40, you can dim it down below half way and it still illuminates really good, the 75 isn't that bad below half way also, but anything else below half way you really won't illiuminate.