Shell is stiffer, takes longer to break in, and won't stretch as much as calf. In other words, it should probably be better fitting to begin with. There are numerous stories of people getting bunyans from shell shoes. That being said, my pair of shell strands fit spectacularly and are probably the most comfortable pair of shoes that I own.
I was surprised that my shell Daltons are lighter than the calf Daltons but this is obviously because the shell version is unlined.
The shell pair weight 2 lb 9.5 oz. The calf pair weighs 2 lb 13.5 oz.
I think that I may prefer the calf version.
Both are pretty awesome, though.