This is an interesting question. I believe that to answer it you will have to look into some other factors as well, such as number of parks in that town/city, income, average age of the residents, accessibility to the parks, weather, fitness attitude and level of the residents, etc.
For me personally and after having done classes in almost every park in Chicago and Madison WI the only issues I found was the weather. Accessibility to and from parks in these towns was not an issue and people are very outdoorsy up here as long as the weather is not misbehaving :-)! I think that being outdoors is a great motivation for people in participating into such programs. As for the community centers, I would have to say it depends. If their facilities are clean, not crowed and the instructors are good then people would be more prone to participate. Plus the cost and fees are much less from those at a commercial gym. I have met some great instructors who work at the local community centers and they do great job in bringing in and maintain participants in their classes.
I’m curious to see what others have to say.