Hello all!  Looking to up to Canada in a few weeks for a few days for vacation. Narrowed it down to Toronto and Montreal. Was curious if anyone could help persuade me one way or the other?  

Flying in,  and looking to stay in "downtown" areas,  as we are not planning on getting a car. Do both places have public transit?  (Subways?)  Approx how far it is from the airport to the city?  

Places to see?  Places to avoid?

First time leaving the US, so I know very little about international travel. Will my cell phone work?  What about ATM cards?

Montreal is a fun city. They practical have a festival every week. I been the F1 race which is next weekend. Make sure you go the week before the race, it's a solid week of partying. The other thing I been to is the Just For Laughs Comedy Festival which is the end of July. It has all first their comedy acts.

Toronto has an extensive public transit system of subways, buses, and streetcars. The airport is fairly far from downtown, I would suggest a cab or limo for that trip. Public transit with your  luggage won't be fun. Your cell phone should work but you need to check your carrier and plan. Voice roaming will be expensive, data roaming will be astronomical. You may be able to switch to a plan that includes Canada for a month right before you leave. ATM card will work if it has a logo that matches the machine (Plus, Cirrus, etc). Call your bank before you go though, to let them know you will be out of the country.

Toronto Pearson Airport had a train that takes you directly from the airport to Union Station which is right downtown. And its cheap! You do not need to take a cab.
The public transit system in Toronto is awesome and there are many options (subway, cab, bus, uber).
Montreal has a bit more historically to look at, but I'd personally choose Toronto. Its right on the water. There are many different areas to toronto as well (culturally).

If you need any specific recommendations feel free to ask!

Recently I've been to Montreal a couple times. Great city!  Flying in again on the 24th. Dig deep if you wanna stay downtown in a decent hotel and book months in advance.  I stay in Laval which is close to the airport and not too far from Montreal.  Lots of things to do and great food everywhere. Be advised that everything is in French but most people speak Anglais.  Check your phone plan!  Like Ohio said, once you get close to the Border and your phone starts pinging Canadian towers they bend you over without lube. Call your bank about your card. My bank just did a new policy about travel outside of the US and certain states. Also, your bank will do a convertion from CAD to USD but will most likely charge you a fee per transaction. You can just hit the currency exchange at the airport and get some Canadian Mylar. Speaking of, hit the Maple Leaf with a laser, pretty nifty.  If you're single, you may never leave. Just sayin. This trip I'll be in Laval one night then visiting Sherbrooke for 2 then back to Laval for 3 nights. It'll be 3 days of business and rest vacation.  Before you book anything you may wanna check the CBSA's web site to see if you may have an issue.  When you go, go with open eyes, an open mind and a closed mouth.