Do I harvest yeast from the racking valve or dump valve?

You could harvest from either the racking valve or dump valve.

It really depends on the particular yeast strain.

I can tell you that with high flocculating yeast, such as most English strains, it's going to be much easier to harvest from the dump valve because of how quickly the yeast ultimately settles into a relatively dense mass following primary fermentation.  

Conversely, with a low-to-medium flocculating strain such as wheat beer yeast, or even some native California strains, you can rotate the racking valve's dip tube in the downward facing position and draw out the extremely clean yeast that resides on top of the yeast cake.  We've done it both ways countless times with a lot of success!