The difference between x1, x1.5 and x2 is the length of the flexwatt. x1 is one foot, x1.5 is a foot and a half, x2 is 2 feet.

How long of a cage are you looking to get?

If it's a 36 long, I think I'd go with 2 feet, considering the temps you quoted. It may be harder to maintain your cool side temp properly with anything shorter in the wintertime.

I'm heating a boaphile 421D (which is 48") with 1.5 feet, and It turned out that was not enough heat with the environment in my house to keep the cool side warm enough for my black blood python. I had to put extra flexwatt in on the cool side to warm it up a bit.

I'd want to know how big the cage is that you're trying to heat before I give you an option on how much flexwatt you'll need.

Btw, boaphile tends up to a month longer than advertised to get the cages/racks to you. They are fanastic cages, but the wait time is horrible (I have almost everybody housed in boaphile racks/cages).