I found myself in a similar position. I applied to Northwestern, U Pitts, and Georgia Tech for a Master's (3 of only 11 schools that offer my program). I heard back from Northwestern way earlier than I expected I would and had a very short time to provide a reply to their admissions invitation. Had 2 weeks to give an answer because they operate on rolling admissions, and on top of that a 'yes' does not secure spot in program you have to pay a pretty hefty deposit to save a spot. I got the invitation before I even submitted the last application (GaTech). Really waiting to hear from Tech because the program would be so much cheaper with the in-state tuition rather than paying the tuition and living costs to move to Chicago. I ended up paying the deposit to have a safety net in case I get a no from U Pitt and GaTech which are both highly competitive programs. I've been going stir crazy waiting to hear back, but at least I know I have one option open, even if it isn't exactly on the terms that I would have it. Now if I am one of the lucky 14 to be accepted in Tech, that's a whole other headache of deciding where to go. I really hope if I do decide to renege on my Northwestern acceptance that they do not hold a grudge; my field is fairly small and I am also considering their PhD program in the future.