camommy is right. The fab day bag is exactly the same as the diaper bag but without the changing pad.
I have this bag in both leather and camo canvas and I love it! It is very floppy, but it sits perfectly on my shoulder when packed, and it has plenty of exterior pockets for sippy cups, keys, cell etc. The inside lacks organization though. But it's so stylish that I kind of forgive it its failings.
As far as the thickness of the leather - Not Rational has two types of leather which are distinctly different. One is the "washed" leather, which is softer and a tad bit thicker. The other is the "naked" leather, which is shinier and thinner. So it depends on your own preference - certain colors only come in certain leathers. All of their bags are lined in canvas though, I think.
If you're anywhere near the LA area, Not Rational is having a trunk sale on 2/17 from 10 AM to 5 PM. Here's the info below. The prices at their last two warehouse sales were AMAZING, but I'm not sure if this is the same deal or not. They are introducing their new spring line.
FWIW, I have NO association with Not Rational other than loving their bags. I hope it's OK that I post this info here.