having some good friends that are locals and expats is important to a good social life here. Limiting yourself to just expats means you are missing out on actually experiencing life in china and are just trying to make life here just like home. And only hanging out with locals may be good for your studying chinese, but you also need that outlet to just relax in a familiar setting

Nice people are nice people. Makes no difference to me. 

I agree with nevermind. Where someone comes from, what they look like, their job or how much money they have has never influenced my choice of friends. I hang out with locals almost exclusively in China but that is because I have not met any expats in  Shenyang where I live when in China. It would be nice to meet some but only for the fact that I could have conversations about some subjects that are not familiar to locals.

It would depend on my mood.  Where I live, most ExPats do hang around a few bars drinking beer and looking at girls walking by.  Not much intelligent conversation besides those two activitioes mentioned.  So, when I am bored to death, I join the ExPats for a while. 
But many times I do go out and enjoy time with locals, since they take me to many different and interesting places, and show me many new facets of their lives.  And we do have fun sharing previous experiences and comparing customs and traditions. 
In a way, I am more receptive to hang out with local than with some of the local ExPats.

It's not really a matter of preference. It's the same as asking if in your home country, you just hang out with your countrymen and not with foreigners.
As anybody else, I like to hang out with people I can connect with. People with who life seems a bit more bearable. If I could connect well with ET, then I would do it too