Both Fourier and Probabilities are extremely useful and relevant to Electrical Engineering, so there is no doubt that you will need both any way.
As you are asking specifically about the relevance of Probability Theory I am going to talk only about it, but as I have already mentioned there can't be a comparison between them as you are certainly going to need both of them, one more that the other depending on what area of Electrical Engineering you decide to focus on. One can write countless lines about where you can use probabilities but I am writing the first things that come to mind.
Probability Theory is one one the most important courses in Electrical Engineering. In most Universities you are taught an Introduction to Probability Theory and Statistics and then in other classes you are learning about more specific areas depending on the needs of each class.
For instance if you are going to choose
- Control Systems
- Signals
- Information Theory
- Communications
then you are certainly going to need probabilities in most of your classes. But that is not the case if you are going to choose Energy or Motors.
(The areas I refer to are just examples and there are a lot more, both in the list that needs probabilities and also in the one that doesn't.)