As it sounds like you will know about the PhD program very soon, I would wait until then (assuming you don't need to let the masters program know before then) and see what happens. If you don't get in, then go for the masters program and if you do get in, well you will have a decision to make. It's no use trying to make a decision that you might not have to make (though with that being said I am hopeful that you will get accepted). It sounds like to me this is just cold feet (I've been there) and really it is only 4 years. Don't focus on the 4 years of misery (and from my experience, grad school isn't all misery) but focus on what's after grad school.
Here's a different way of thinking about this...let's say you do get accepted to the PhD program and you turn it down, do you see your future self regretting this decision? No matter how frustrated I get, and how many times I want to quit my masters program and just forget about a PhD program, I know that if I give up on this I will regret it.
I agree with the above - go with what feels right, don't feel that you have to put pressure on yourself/time limits on yourself in terms of achieving X by Y date. If you think you'll enjoy it, what's to lose? If in doubt make a list of pro's and con's, that always helps me
