GC or Pro landscaper
If you are putting down a patio made of concrete pavers over a compacted subbase, gravel and sand, the whole thing sloped appropriately for your site, then I'd go with a pro landscaper - and specifically one that has certification from the ICPI (icpi.org).
Not that this may not be doable by a good GC - even a good DIYer with tools, but the foundation is the most important part to get right, consistency and slope-wise. I mean screw up the stone pattern and you can correct it but screw up compaction of the gravel and you're heading for troubles that can cost twice as much to repair.
Again, not putting down a good GC - but a certified cement paver guy has to know this stuff for a living. Would a GC just go with 6" of one- time compacted subbase (ie gravel) or would he know enough to add 2" for this climate and use the appropriate gravel compacted the right number of times? or would he suggest 8" to be on the safe side but that he's also like our clay base to be compacted to the right consistnecy before he even puts down anything?
A pro landscaper earns his money in the preparation mostly and less in the actual laying of the stones. Usually one guy lays the stones so comparatively, it is a small part of the overall job. That you can do yourself - and he wouldn't mind.
These are all general questions that may all have the same answers in your area - but that also might have different ones depending on exactly where you are in the GTA. And exactly what stones are you considering? - that might enter into it too...
All this to say that a pro landscaper with training either with a paver supplier or certification from the ICPI is a good bet, IMO.