Tim+ was thinking very hard :
Definitely. With the appropriate regulator, heat output doesn't vary much
but butane, per cylinder, per therm, is considerably cheaper.

Cheaper, but not considerably so. Using the correct regulator pressure
for the gas, there should be little noticeable difference. There is a
slight difference when I used to bother to swap between butane and
propane using the compromise between the two gases bulkhead regulator.
Now I just use propane for all seasons.

The petrol stations which supply car type gas, vary the butane/propane
mix depending on the season.

As long as
you don't need cold weather performance it's cheapest option.

That is about it, just remember butane is near useless -10 C.