Nav is available on the sport or GT. You can get a TOMTOM or Garmin for $300 at Target if you want nav. Yeah, It won't be as clean as oem, but for $1700 for the upgrade? I would have to say skip it. I have a pretty stock GT, other than Sirrius and auto dim/compass mirror, and for that $1800, it really is money well spent. The HID's alone are worth it. In the long run, so are the leather (ish) seats. The rest of it make daily life with your ride pretty painless.
i looked a the sport for about 3 minuets, and saw my GT soon after, and threw down the green as soon as I sat in the GT. It is personal preference, and the only way you can make this choice is to drive both. You will have to live with this car, not the rest of us, so choose wisely. All of us here think you are wise for looking at the MS3 anyway, so have fun with either the sport or
gt. Good Luck from all of us here!(drive2)