Tell Mississauga you'll let them know if you can take it, Wednesday afternoon, that you have other people you need to consult with :-) Hopefully they won't call until Tuesday or Wednesday so you can easily tell them you'll call back the next day.

You could always take the Mississauga job and go to the interview Wednesday and back out before you start if necessary; that happens quite often I think? You're allowed to make a mistake, have a better choice for you come up (they do it with people, if someone accepts and then drops out they try again with their second choice?); it happens all the time with college applicants; they know you're going on other interviews and you can use that to your advantage, especially with the job you really want by hinting you have Mississauga waiting in the wings so are a very desirable lady and they should make it worth your while to back out of that