My gut says $8000 and 25k for the leaf.
Lower miles may mean a better body condition which if you don't drive a ton, makes sense.
When would the extra range matter to you with a Volt? are you pushing the envelope frequently in a month with the Leaf?
-- note, I have sat in (owned or leased by friends) a New Leaf (small car, family car), and a NEw Volt (seemed quite executive to me, relatively speaking)... but that finish and size level difference would not matter to me, as I would be driving it myself, with one other adult, not driving clients, etc. It would replace a 11+ year old Mazda 3 in my case, so either car works, and I would go with the lower price for the value win..
The owner of the leaf loved the reduced parking, rebates and no fuel which made the car almost free for him, but could only drive it home to work (lived near sacramento, so other cities too far).
The owner of the volt drove it everywhere without pause, and thought it equally nice to his friends Mercedes for driving to / from work, 1 and 20 minutes each way, around 30 miles.