As has been said, it depends on the distance to the destination and the main difference is the guaranteed delivery time, not necessarily the actual delivery time. For instance:
Package going across Ontario ( K2G5Y7 to N3C3T2 )
Expedited: $8.61 (2 business days guaranteed)
Xpresspost: $10.40 (1 business day guaranteed)
Priority: $20.32 (1 business day guaranteed)
Notice there's barely any difference in price between Expedited and Xpresspost? That's because the destination is so close (~500km) and it is a relatively populated area (Cambridge, ON), so the speed isn't that different. We've shipped many packages with quotes similar to the above, but Expedited Parcel still arrives in 1 business day.
On the other hand...
Package going from Ontario to BC ( K2G5Y7 to V0E5N0 )
Expedited: $17.22 (6 business days guaranteed)
Xpresspost: $31.47 (3 business days guaranteed)
Priority: $48.93 (2 business days guaranteed)
Now things are a little different. Xpresspost is nearly double the cost and Priority is triple the cost. The destination is much further (~4200km) and it is a lower populated area, which adds to the delivery time. Xpresspost and Priority will go by air whereas Expedited will still travel via truck. However, it is worth noting that the "6 business days guaranteed" using Expedited Parcel could actually arrive in 4-5 business days, depending how busy Canada Post is at the time. So, you might only be saving 1-2 business days of delivery time by upgrading.
Moral of the story, only pay for Xpresspost/Priority shipping if you absolutely need the package to arrive within the guaranteed time. Expedited will say it is slower, but it isn't always that much slower. We primarily use Expedited Parcel because it is the cheapest option available for shipping packages within Canada compared to any other courier, and the packages almost always arrive on or before the guaranteed delivery time. My only beef with Canada Post is their ridiculous $10 oversize fee for packages 40" or longer.
Oh, the box is 39.5" long? That'll be $15 please. Wait, it's actually 40" long? Oh, then that'll be $25.
