OK, this part can be confusing, but it's actually rather simple. First of all, because you're a US citizen your application will go to Buffalo - regardless of which address you use. That's because all US applicants are processed through Buffalo AND any foreign national who is initially admitted to Canada for a period of at least one year is eligible to use their Canadian address as their residential address - and those applications are also processed through Buffalo. So, say you were from the UK and you were initially admitted to Canada on a one year work permit - you would qualify to use your Canadian address and have your application processed at Buffalo. Being you're from the States and you were admitted with what I'm assuming was probably a one year work permit, you could use your Canadian address as your residential address and your application would go to Buffalo . . . but it will go to Buffalo anyway. The only difference is whether it will get transferred at some point to one of the other US visa offices - and that can be an advantage. First, sometimes Buffalo transfers applications when they get backlogged to one of the other State-side visa offices (Detroit, NYC, Seattle or Los Angeles) to expedite processing. If your ap is being processed based on your Canadian address, it won't get transferred. In addition, if your ap stays in Buffalo and an interview is required, it will be held in Buffalo. If you're in Ontario, that could be good. If you're in BC, not so good. By using a US residential address, you have the option to have your application transferred for an interview to a visa office that you designate in the "Interview Location Preference" form in Appendix A of the US Region Specific guide. You also benefit from them sending your ap to the visa office closest to your US residential address if they do so to expedite processing. You can always use your Canadian address as your contact info - so you don't have to worry that if you use a US residential address, they'll try to contact you there and not find you. They won't. There is no residence requirement to outland aps - you could be in Siberia and still have your ap processed through Buffalo because you're American. Your ability to be in Canada is based on your work permit - and you're eligible to be processed for PR outland, as a US citizen through Buffalo, regardless of where you're living now. Hope that makes sense and answers your question.
