What are the class structures like?
Is it done on engine size or power to weight ratio?
If you are un-decided you're best bet would be to go and speak to some drivers in both. Championships are very good at selling their series to drivers, but at the end of the day, get some advice from the consumers (drivers already racing) as they will let you know stuff thats not normally easy to find out, politics etc.
Like has been said, how much money you want to spend also comes into it. Items like tyres, can be renewed every race in some, but that soon increases cost, and to be on the pace you would need to do that.