Q is a measure of quality. CRF is constant rate factor. The smaller the number, the better the quality, the bigger the filesize. (18 is usually absolutely sufficient to ensure good quality.)

Using CRF means that you are choosing to output a certain quality (in terms of encoding) at the cost of unpredictable filesize. When you use a two-pass mode, you can select the filesize that you end up with, but the encoder can only deliver the best quality it can for the filesize. At the sizes that you are talking about, you might not notice much of a difference.

I agree that the screenshots look great! :-)