Really they are both very similar though OF uses more cross platform libraries to keep things consistent between each where as Cinder uses platform native code so you will probably see a slight performance boost when running a similar program. I personally think we have a more active community of people creating addons and that OF is a bit more approachable because it hides away somethings like namespaces and also we use header classes to try and keep things organized. That's certainly up for debate but I like to use headers as a way of organizing things as having it all in one file can get sorta muddled and messy when tinkering. I also think we have a stronger naming convention so its easier to know what something does based on the function call alone (less domain specific jargon)
From my own experience I have seen more people using openframeworks, I just am returning from MIT who uses it to prototype a lot of random things which was pretty cool.
