As far as I know, they do not have a detention capacity that's any better than what the Canadians have. In fact, they're probably using the same detention capacity at Kandahar airfield. But they do have a stronger agreement that ensures that any detainees they transfer can be followed up by right of access--not by discretion of the Afghan authorities, but by right. That means they can go into these prisons that Alex has described and ensure that our detainees are not being mistreated in any way. That is the crucial difference.
The Canadian agreement does not provide for follow-up and leaves this to the ICRC, which will not, in practice, report back to us. Therefore, it is essentially allowing us to wash our hands of it, which under international law we are simply not allowed to do.