Well both suck but they give someone valuable experience. One does not need permanent residence so being booted out after 6 years or less or not being able to apply for PR is not necessarily a bad thing. It is mostly some work permit holders of a specific nationality who are absolutely ready to do anything for the 'green card' ot whatever it is called now, at least in 1998 they called it that. Most Canadians would be happy to get the experience and move back if they need to.
The main downside is that you cannot do anything else ie work on the side etc. The second being that no unemployment benefits if the job is gone. The UK offers a better deal I think, no ?
All over the world, work permits are temporary and do not necessarily lead to permanent residence. For some strange cultural reason, I see a few nationalities desperate for PR in the US.



