I strongly suggest that you go in person. If I were you there is no way I would put my passport in the mail with a trip in the near future. The office is only open to the public from 8 until 11 (or 1, I can't remember exactly) so they have plenty of time left in the workday to get you processed and out. I can not imagine what extremely rare circumstance it would be if they did not give you your passport back that day.
I took my passport in person. I suggest you plan to be at the office before it opens (like 7-7:15 am). There will most likely already be a line forming in the lobby. The guard will ask why you are here and check your passport. He will escort about 10 people up at a time. When you get there, they will ask for your cell phones and tell you which window to line up at (depending on your purpose). When it`s your turn you will submit your passport, pics, and PPR letter. I think they asked some questions too. Then you are sent to the waiting room to, you guessed it, wait. It takes about 2 hours. Meanwhile, you can watch people going in for interviews, read a book, etc. After about 2 hours they start calling several people to one room. You line up and they give you back your passport and some papers explaining SIN, health card, etc. Then they tell you to wait a few hours before attempting to cross the border while the computers are being updated.
Go to lunch, do some shopping, head to the border and land. That's it. Landing takes about 15 minutes if there is no wait. They take your papers, passport, etc ask some questions, confirm your name, contact info etc, do some stuff in the computer. Then you are off to customs to declare any goods to import, and then you are DONE except for coming back here to report your experience.
I think you have a lot less worry about if you go in person. Congratulations. Allison