Don't want to be a downer, but what do you think are the compelling reasons to upgrade at all?
You don't get any new functionality over your existing unit, just a faster processor so it is a bit more responsive in the UI and route calculations.
Your C unit can run the same firmware as current systems if you havent already upgraded it so that you get RDS and make voice guidance which would be the only functions that you may not have had on older firmware.
Q units are designed for Chinese domestic use. If you are set on upgrading, get a P.
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