@Whistle_Pig Are you certain about this? 5-19 is a mountain hex, and unlikely to be mined any time soon, while 5-15 will be lost if you build a mine on 4-16. Apologies for second-guessing you.
EDIT: actually, this is probably too much from me. I have gone ahead with the explore, and will refrain from suggestions in the future, unless suggestions are requested.