I did some stonework on my current house a few years ago - at the time the stonemason offered to do the dry finish for almost the same cost as mortor finish because his stonemasons could do ''dry work'' in the rain.
(i was getting it done in January) - it may also have had something to do with the fact that he had promised me he would do it during the previous summer but got delayed...- anyway, I went with the mortar finish as I was a bit concerned about dampness with a dry finish.......
Maybe you should ask your stonemason to take into account the all-weather aspect of ''dry work'' !!