StackOverflow is definitely more important right now.
But one thing I think they're messing up is the process for launching new StackExchange sites. They're trying this thing to qualify new sites to ensure they have critical mass, but I think it's too restrictive and hurting their growth.
As an example, I "pledged" to contribute to the AI stack exchange site if it were to launch. When it launched as a trial it received maybe 100 questions and some good answers in the first few weeks. I got answers to all my questions - good ones which pointed me toward research I hadn't found in Google etc.
But a week later it had been shut down and they emailed everyone saying it didn't meet minimum activity thresholds. Those questions and answers are no longer available on the net.
The question is whether a StackExchange site with relatively small activity is still useful. I think it is - at least it's more useful than not having them at all. Who is to say a small site couldn't become much bigger over time.
Reddit has done well letting users create any subreddit they want. There is a diverse ecosystem of subreddits out there with varying levels of activity. Keeping the long tail of subreddits around to see which flourish doesn't seem to hurt anything - so I don't understand the StackExchange approach to this.
(It reminds me of Wikipedia deleting obscure pages also - what does it hurt to have them?)