Here's my take on it, it lines up with some other folk's websites (that sell both the on-off and the proportional type).

An off-on type of t-stat is using drawing 100% of it's current load when on and 0% when off.

The Proportional type draw 100% of it's current load all the time and and deliver some portion of it to the heating element.

I don't think the simple resistor-type heaters that we use don't really have a "Startup penalty" like turning on certain types of lights with ballasts or inductive motors that have a start current to get rolling.

So, to my simple mind the on/off type are going to use the least energy.

Now, you can start a whole other discussion around what type is BETTER for your snakes.