I spend time bouncing back and forth between Canada and the US, but am a light user. In my case, going prepaid made the most sense since the bundles never expire and there's no daily charge. Never have to worry about whether answering a call is going to cost $7 or more.
You do have the option of the old 45 dollar add-on for more than one month at a time but you must speak with a rep to have it added. If you only need it for one month then it won't be a viable option. I'd go with the above recommendations otherwise and personally prefer local Sims as the Mobilesyrup link Bob provided give info on.