The 40# would be fine but I have found over the years I keep going bigger with my TM and am happy every time I do.
Get bigger battery's and/or carry more than 1 which I usually do when I fish alone and can stand to have the extra weight.
I have a 55lb on my Bass Hunter EX and its great.
The thing with draining your battery is this, top speed does the damage. It drains the battery way faster than lower speeds. So get the 54 and dont run it on max unless you need to and you will be fine.
I like the extra speed when I need to get to a boil, get out of the way of bad weather coming or a dumb boater who thinks hes king of the lake bcs he has a 200hp motor and I have a 10 foot plastic boat ha ha.
With the 55 and just me in the boat I can get close to 5mph. Thats 1.5mph faster than my 40 did.
In summary, if you get the smaller motor, when it goes bad in a few year, you will get the bigger one and wonder why you were not running a bigger one the last few years.