For people applying inland, it's always advised not to travel until they get their PR, unless they really have to. The reason for this is that if you're denied entry to Canada upon your return (for any reason at all), your application is automatically cancelled and you have to start all over again.
Since you're American, your process will most probably be faster than average so your wait will be shorter in that sense as well. Whatsmore, outland American applications are processed within 6 months these days so outland is usually a better option for you guys too. I would also wager that your chances of being denied entry to Canada as an American are lower, but all it takes is one immigration agent having a bad day to ruin yours.
So basically, the standard advice is if you really don't have to, don't travel.