In terms of size and population of the region the airport serves and the traffic that Pearson itself experiences, I really think that the Weston needs to be able to offer a number of choices for destinations and speed of service.
In 10 or 15 years time (about how long it will take for any line to be constructed after EA's, planning, and construction), Pearson will probably have passenger traffic of 35 + million people and the Golden Horseshoe will be approaching 10 million people (about 7 million for the GTA alone and a little over 3 million for Toronto). These are not small numbers and in terms of comparisons that may be roughly equivalent the one that comes to mind is Schiphol (42 million passengers in 2004) and serving Amsterdam, surrounding metros and other cities in Holland, the number of residents it serves is roughly equal, if not a little higher than the Golden Horeshoe will be in 15 years.
Schiphol is served by NS-Reizigers, which is the passenger division of the old Dutch Railways. The station itself is very minimal, directly under the airport itself. It connects to Amsterdam Centraal primarily but has trains that make local stops as well and will also be served by the new HSL that is opening in 2007.
What makes the Dutch example ideal for Toronto is its simplicity and flexibilty. All trains run on traditional rail (designed for passenger and freight needs) and of course electrified. However by using a variety of rail stock it becomes very easy to offer local, regional, or intercity service. Below are some pictures of the stock that is used on the Dutch railway, all of which I do believe (except for the ICE) serve Schiphol.
EMU DM'90 Buffel. Primarily for local service (ideal for Union to Pearson local service).
EMU set known as Koploper. Primarily used on intercity trips.
Newest EMU NS Sprinter. Largely used for commuter and regional service (and some local I believe). Very quick trains. Can reach 125 km/h in 75 seconds.
Older model NS Sprinter.
NS Sprinter next to a German rail vehicle (possibley a Talent).
EMU Regionrunner Commuter trains. What an electrified GO train might look like (and made by Bombardier).
EMU ICE for all your long distance, high speed, intercity needs.
What is of little concern is who actually operates on the tracks. If GO, VIA, and the TTC want offer various services, great. If Mississauga wants to continue its rebelious ways and offer its own rail service to Pearson, go ahead. I dont even see a problem with a private operater running a $20 Pearson to Union express service so long as everyone else can operate too. But for a city and region the size of Toronto and the Golden Horeshoe respectively, I really hope the solution is reflective of its immense and growing size.