Choosing the default application does not stay as default after reboot
I was doing it the wrong way like you try to for a long time and also have retreated to using RCDefaultApp in the past... but there's actually a way to do that natively which works.

Do the following:

1) right click your file 
2) choose "Get Info"
3) in the popup find the "Open with" strip (this is by default closed) and open it
4) from the drop down choose the program you want to open that type of file
5) click "Change all..."


And that's the correct way to do it