I don't have those chukkas, but recently bought a pair of string loafers on the same Xim last from Epaulet. The sizing advice on their webpage is pretty spot on, so I would imagine it's the same for these chukkas.
I guess if you plan to wear them in the fall/winter, the pebble grain version makes more sense since it has a rubber sole. I'd pick up a pair if I don't already have enough too many chukka boots as the price is insane right now.
+1. I have the brown pebble grain chukkas and love them. I'll wear them into the late fall and early winter here in the south, especially given the pebble grain and rubber sole. They are perfect with jeans and a casual setting.
I'm using self-control on my spending right now and waiting for the new Walts and the speckled chambray shirt before deciding what to grab. I really like those grey Rivets!!