SaugaBoss -
Here's my two cents as a long time hiring manager...
If you decide to go for the contract, tell them you're pregnant before you accept. Say you're still VERY interested in the role and if they're willing to take you on knowing you will be leaving in July, you'll be thrilled to work for them and will give it your all. One year maternity contracts are very hard to fill with good people (I know from first hand experience). So the worst thing you can do to a hiring manager is not tell them you can't do the entire thing and suddenly drop this bombshell a few months in. This is the sort of behaviour that's regarded as professionally unethical and can get you blacklisted from companies.
While Toronto may seem like a very big place now, it's amazing how small it is when it comes to the working world and checking references with old employers.
So I would recommend that you go for what you want! First talk with Maple Leaf and tell them you're pregnant. Like you said in your original message, there's always a chance they'll say yes. Tell them you have another job lined up but would prefer to work for them - and tell them why. If this falls through, then you can always go for the call centre roll.
Good luck!