I just got an a6000 and I love it. My needs in a camera might be different than yours though, but it's as quick to focus as they say it is. I can't wait to get out and shoot some skateboarding with it.
Here are a couple of sites to compare the two choices you mention.
http://www.dpreview.com/products/com...cts=sony_a6000
http://snapsort.com/compare/Samsung-...ny-Alpha-A6000
One thing to mention is the a6000 has an electronic viewfinder while the NX300 doesn't have one at all. That can be important if you're shooting in bright sunlight since the back LCD can be hard to see.