+1 to what daverobev said.
TFSAs are tricky beasts since the taxation agreements haven't necessarily caught up to them yet.
As for which, look at the fees but also look what other investments you have. If you have them predominantly in Canadian equities and bonds, go for the US. If it is predominantly US-based, go for more at home. What you want is a good balance of items so that if one area dips, another one counterbalances it. The past couple weeks makes for a good example of this. If you had mostly Canadian investments, our dollar has been tanking. Having some US-based investments (in USD) means if the CAD drops, the value of the investments in the US funds increase (for Canadians at least).
For me, I try to be mostly international, a good part US and then a reasonable bit Canadian (like a 50: 30: 20 ratio). The Canadian economy isn't that big, so I don't try to overvalue it.