I think they can both work, so this is an issue. Both of them are a lot better than nothing, and to be fair, the current government has introduced something that is a lot better than nothing. The current government's new rules really would restrict emissions, although in fact they are not quite either of those two things. There are many mechanisms that you could use.
I favour a tax because of the extreme simplicity with which you can implement it. Cap and trade generally requires you to keep track of every facility's emissions, and there are a bunch of complexities that come with that, especially regarding new facilities, so this often gets kind of sloughed off.
If you look at the current rules—and it's no fault of the current government, since every government that tries to implement this sort of system has a problem—you have to deal with what happens to new facilities. The current rules do this common thing of demanding the best available technology. The fact is that's an excuse for lawyering—with no offence to lawyers in the room—because it's not objectively possible in the real world to know exactly what the best available technology is, especially for something complicated like oil sands. So basically it's an excuse for backroom negotiation.
The advantage of something like a tax is that because carbon is a conserved quantity in the economy, it's actually pretty easy to put a ring-fenced tax on the economy with very little extra overhead in terms of accounting systems. You're sure of one answer that you're going to get, so I favour a tax.
I'll say one more thing. In backroom conversations I've had with people in government, NGOs, and the oil companies, I routinely hear people agree with me that a tax is the best thing, but it's politically unsellable. I put it to you folks in the room who are politicians that if a lot of people in the backroom are agreeing that something makes sense, and they say that it's politically unsellable, we have to think about how to sell it.