It really depends on the weather in the days prior to and during your trip. Of course, there is no way to predict that. It also depends on how warm you like your pool water.
If you prefer warmer water and want to be safe, go for the pool heat. If you're willing to wait and take your chances, I think the companies may let you call when you get there and have it turned on as long as you pay the fee. I would ask about that before you decide.
Steve