Frankly I think the best idea is to simply read IRS publication 519 (and the instructions for 1040NR). It will put you to sleep but it's what accountants use so why bother with them
Bear in mind you're supposed to state on your W-4 that you are a non-resident alien when you start work if you file as non-resident, which is why most people don't because it causes a problem for the employer (supposedly, I take it this means they can't add because it's really simple).
However from a paperwork standpoint it's definitely easier to file as non-resident, although you will probably pay more tax as Canadian rates are higher (and you have to pay the difference between the income tax rates). But it is cast-iron proof you have "non-immigrant intent" for TN-1.
On the US end you basically just file a 1040NR and 8840 (or 8833 in some situations) with your W-2 and any other paperwork you need to claim deductions, that's it. Because you're non-resident none of the other paperwork like the 8891 comes into play. Then you simply file as normal in Canada and claim a foreign tax credit for the US income tax you've paid.
This CRA publication explains it in fairly simple terms:
http://www.cra-arc.gc.ca/E/pub/tg/p151/README.html
Note that there seems to be confusion in the IRS as to whether you use Form 8840 or 8833 to claim the tax treaty exemption. The CRA publication (written by the IRS) says to use 8833 if you meet the substantial presence test, 519 says to use 8840. My take on it is to use 8833 only if 8840 doesn't cover your situation.
8833 can be used in any situation 8840 can be used; however 8840 can only be used in situations that apply to non-resident alien individuals (8833 can be used by non-resident corporations, trusts, etc.)
Steve.