They are very different and I would suggest to go with what is closer to you if you have been to neither.
Southwood is much tighter and technical. There are some bumps and frost heaves in the corners and sand is often found late in the season. There are also a couple of really rough railway crossings which you should slow down for. But overall it's a really fun road and the fall colours are amazing along it.
The 507 is a much faster moving road with wide open sweepers and no tight technical turns. You can get yourself into trouble if you are going too quick, as there are some intersecting forest roads that slow moving vehicles might turn out of. North of the 507 are also some amazing roads in and around Haliburton and Bancroft.
Since I live in Mississauga and like the tighter stuff, I would prefer to go with Southwood and then also hit up the roads in the Muskoka region for some added fun. I haven't been on either since the spring, so others can chime in on the current road conditions of both.
