I have a Box Kelly and also just bought an Evercalf Constance a few weeks ago. Having compared both leathers, I find Evercalf quite similar to Box in the smoothness and shinyness characters. The main difference for me is Box is a lot more rigid and structured while Evercalf is so buttery soft! Both will develop light scratches quite easily. I suggest you really give Evercalf a try in store.
Overall to me the 2 leathers are not too different but the style of Kelly they come in are very different. A Box Kelly will look more structured and will not look too slouchy over time while an Evercalf Kelly will look very slouchy over time due to the softness of the leather.
Below is a pic of my Evercalf Constance.