does GC holder need passport to fly/drive b/w US/Canada?

[Steven]

Be careful using multiple passports, I know a couple of people who've gotten grief doing that. If you travel from the US and you're returning to the US, use your US passport always when you book the flight, only use one of your other passports when you actually get to immigration at the other end.

I have to say I think US consular services are superior to Canadian ones usually (hard to generalize, depends on where you are and what the problem is) so you're better off going as a US citizen.