I tend to rent more in the winter months, or if I know I am going to be on bad roads. I have family up in northern ontario, it's only about a 2000km round trip, but I know at least 100kms or so is on gravel roads - something I'd rather not subject my own car to, so I usually rent.
Also, in the winter when I know the highway is going to be a salt bath for the entire journey, I'd rather rent and not worry about the affects of the excess salt on my own car.
In summer and on paved roads I'd rather drive one of my own cars, regardless of distance mostly for the fun factor. I also feel slightly out of place in a cheap rental....While old, my own cars are a reflection of me, and I am comfortable in them....I feel a little awkward driving around in a Base Nissan Micra, or whatever.