Definitely avoid the roaming fees.
Buying a US SIM card is in my view the most convenient.
- You can call people in the US from a local number
- You can use data on your phone to access things like maps, etc. without being ripped off
You will need to have your phone unlocked if it isn't already.
With Bell Canada there is reportedly a charge of CA$75 for unlocking (source). There are numerous services online that will do this for you for cheaper. Here's one example: http://www.unlockallcellular.com/
The only real downside to getting a new phone numbers is that you will need to give your family this new number to ring while you are abroad. I would recommend buying your SIM card before you go to the US so that you can have a number to give people before you go and also so that you don't waste time looking around while you're in the US. That can be done through any one of a number of resellers or there are some good deals on eBay for prepaid SIM cards too.
@jpatokal already mentioned the possibility of using Skype, which is good except for the fact that you'll be needing to find WiFi to be able to make/receive any calls. You could try to work around this by just buying a SIM card with only data and no call credit. Then use that data to make/receive calls through Skype. You could even set up a Skype incoming number so they can call from a normal phone line. Extra costs would apply to do that course.