I have not sailed the 2008 Rio S (175 liters; 269cm length; 76.0 cm width; 56.2 cm OFO 14.1 Kg. weight) so I cannot tell you from direct experience what the differences to the
Rio M might be.
Yes, getting it into your car is a good thing (are you sure you'd rather have a wet board inside on your new interior than outside (where it will dry quickly) and since you mostly sail on fresh water the affect on your car's finish would be similar to rain).
Ok, now let's discuss the stability and volume issues.
The dealer may be correct, the wider 2007 Rio S is going to be more stable for sure
The 2007 Rio S (150 liters; 235cm length; 85.5 cm width; 56.0 cm OFO; 11.95 Kg.)
Is going to be more stable that either of the 2008 Rios because it's wider.
At your weight, you will have no issues with volume/float as long as you figure out where (fore and aft here) you need to stand to get your weight matched up with the point of max. bouyancy.
What boards have you been sailing on previously?
My suggestion to look at the Rio M was based on getting a board that fills your needs.
Stable, goes well in light winds, but with plenty of planing performance down the road.
For someone your size I still think the Rio M is the better choice because it's going to give you the easiest learning curve and the best light wind performance.
The Rio S might be just as good, or better, once your skills develop to the point that you no longer need much lateral stability, but in really light winds my guess would be you will give up some of the "glide" the Rio M would provide.
Hope this helps,