I just went through a job search. I was sending out resumes for 3 months and then literally in the last 2 weeks I had 3 different job offers that came to me and they all wanted to know if I could start working yesterday.
First off, if you're looking to be hired by an academic institution, they all are going to have to through their human resources departments (usually). This is very frustrating because regardless of if the PI or department really wants to hire you, they usually have to go through the ridiculous process that HR establishes.
My advice to you is to figure out what job you want. I personally work best if I think out all the possible situations in my head and then try and see what I would do regardless of the situation. I'm not sure if you've been honest with the job that you'll be hearing back from on Monday, but if not, try and stall them until at least Wednesday when you will have a better idea of what you want. Then you can take it from there. It's pretty stressful, but usually if someone wants you, they'll make it very clear that they do and then usually it's a matter of taking a leap of faith.
Good luck and feel free to PM me if you have any questions!