Personally I would run a 17" for two reasons. In most cases the tires are less money per tire and usually around the 18", the tires are a heavier load range like load range E. Which means a heavier, often times more stuff tire and can decrease gas mileage and power.
If you are staying with more of a street tire then the load range of the tire may not be an issue. Double check before you but them.
You may also find a larger selection for tires with a 17"
If you are not planning to lift it or go off road much then I would stay with right around the same tire size maybe a size bigger.
This is just my personal preference.