I would strongly suggest the QA bucket setup regardless of the year of the machine.
That said, if both have 0 hours, it depends on how much the dealer wants to move the machine. Model years make less of a difference, hours is the big driver. Assuming they didn't change anything from 2015 to 2016...if they did, then that's another story.
Between paint fade, minor rust from the machine sitting for the extra year, possibility of condensation in the hydro, engine oil and fuel from sitting, etc....I would expect them to mark down the 2015 a few thousand $ at minimum...but that's just my guess.
you buying cash or financing? Both machines should be eligible for the Kubota low rates...
Good luck, like I said you WANT the QA setup either way.......its more expensive to try to add later.