look how much sassier you've gotten

Dam right!

The difference in styles is so marked. Strangely, it seems to me that the 2013 version looks slightly more polished than the 2015 version, but the latter looks more modern.

Could you explain why 2013 looks more polished? i cant see it.

I'm really not sure. This is what I'd call a je ne sai quois -- an undefinable quality. It may be that the 2013 image looks a little more well proportioned, and the shading seems more appealing. This is all just personal opinion, of course. Your art, on the whole, has taken massive leaps since the first picture I saw you do (which was my chakat mate Silvertail -- I had it framed) at the con a few years ago.