I'm bringing this thread back from the dead again, as I just now noticed it.
A Newgrounds plugin for C2 sounds like a great idea.
I'd already been considering Newgrounds but seeing this old thread did cement my decision to release my new little adventure game (Spiral Skies) there.
So, thanks for the advice.
I did look at Alexa rankings for the two sites:
Kongregate is currently the 1608th most visited website on the internet.
Newgrounds ranks 4100.
I also checked SimilarWeb's rankings, which placed Kongregate at #1075 and Newgrounds at #2161.
They also estimated 32 million visits to Kongregate and 18 million to Newgrounds, in the last six months.
Kongregate has about 106,000 games.
Newgrounds has a total of about 3300... but they also have some categories that aren't games. Videos, for example.
I examined the ranking systems for games and I think it's significantly trickier to get noticed on Kongregate if your game isn't above 3.5 stars (A lot of games with lower user ratings fall through the cracks) but that Newgrounds makes it a lot easier for games with middling ratings to get discovered.
My conclusion? If you have a truly exceptional game, and are really confident you'll be above 3.5 stars, Kongregate may generate a ton of attention for your work. Otherwise, Newgrounds is more promising. It is almost as popular and there's much less competition in the games category on Newgrounds right now.
The Pareto principle, while a good rule of thumb, doesn't always apply. Kongregate heavily biases its results towards the most popular games, the 1-3% of the games submitted that can break through the clutter and grab attention and ratings good enough to be listed highly on a category.
Newgrounds is a better option for decent but not outstanding games. You might actually be seen there with games that are good but not great.
I'm releasing [Spiral Skies] on both in the next few days but now I'm thinking Newgrounds is a safer bet as I'm not certain that my project will climb above 3.5/5 on Kongregate.
It could happen, but there's always a lot of cynicism and negativity on the internet so it is a hard thing to make something that is so good it will rank well.
Some links and pics related to Spiral Skies: