What was your previous setup? At 40 watts (although I know they're internal battery, at least I think they are and maybe function a bit differently) I would tend to think that a MTL tank (less wattage, less vapor, less battery needed) would probably work best.. Without knowing your vaping style though, it's hard to give a recommendation, I mean if you prefer things less restricted, you may be able to get away with a slightly lower resistance coil in about the 0.5 range vaping it maybe at 25 watts.
What did you used to use on there, and what tanks do you currently have available. Is this temporary until you can replace your new setup or are you planning to purchase a tank and have it as your backup?
Anna