Is the 109 especially busy?
It can be, packed in fact, at least for the evening peak. It's one of my saving graces to get to Dundas West when travelling in from the K/W/Cambridge/Guelph area. Even though the Metrolinx regs state they have to allow you (gist) "the cheapest way to travel *by any route* to your destination", they won't honour transfers as an open ticket if you transfer to UPX at the airport via one of the two buses that serve both Square One and Pearson.
So I'm forced to either go completely downtown to Union, and then back northwest on the UPX to Bloor to make it all one fare, or use the TTC, and/or MiWay. And that means the 109 to Islington. It's the fastest way I've found from having to do it hundreds of times, mostly with bike after extensive cycling. It means you don't get co-fare on TTC as well as paying fare on three systems.
One of these days I'm going to test Metrolinx' written bylaw (Part B) in court by insisting that UPX hounour my open ticket for one fare through Pearson the way they do through Union. And God help me from having to ever take the 21 H or P or whatever the hell the latest milk run is down to Union from SqOne.
Taking the 103 (?) down to Port Credit means having to buy a GO paper ticket to not get charged twice to get on the LSW and onward, and also means that since you won't be using Presto to tap your way there, you have to show that ticket for the MiWay driver to manually enter age concession and co-fare so you don't pay full MiWay fare.
Some 'integrated' system...
On the 109, I've learned to change at Winston Churchill, it's just a few steps down the platform as opposed to the utter madness at Square One, and less chance of missing the best connection.
Addendum: Of interest is that half the bus load of the westbound 109 going to the subway disembark at the Aukland stop, and walk down to the Kipling station, rather than continue on to Islington. Next time I don't have my bike on the front, I just might try that, the traffic along Dundas and down Islington is inevitably a hassle.