If you are thinking about buying a new mod anyway I would plan to buy one with more power or even TC.
The reason I am saying this is because it will leave you with more options for later times and the additional price for a device with more power is rather small.
If you are on a budget you can go with the 40W device (it will suit the subtank).
But it usually happens that in a while you want to buy/try some new vapor and if that needs more power or TC you end up buying a new mod at that time too, having the previously bought one lying around.