Do they offer free returns on sale items? If yes and if you have the fund, I'd suggest order all three models to try them on to see which one suits your lifestyle best, or if there is a store in your vicinity, maybe you can go there to look at them?
Maybe I'm biased but Darley (the rectangular one) and Lily are my absolute Mulberry favourites. I think the medium Lily (the 21cm at base one) might hold more than the smaller (mini) one. However, the medium Lily's base can sag with use as it is not as supported as the Darley or Sadie, so it may detract from the charm of the bag, but of course only if that bothers you.
Another thing to consider is the strap. I personally think the Sadie would be a bit more comfortable to carry and a bit more functional. Plus I have a feeling that if I had a Lily I would have to baby it more than the other two. (Having said that, I'm still on the hunt for a Lily - that bag is just my aesthetics)
I hope you will finally find the bag that is most suitable for your need!