Need more information. Is this a brand new 2016 in dealer inventory? Is there some reason you would rather have the 110 Twin Cam instead of the 114 M8 engine? Do you have someone wanting to buy your 2015, or were you thinking of trading it in?
If this is a brand new 2016 in dealer inventory, you need to understand that it will depreciate a bunch the moment you sign the contract and ride it off the lot. I have no idea what, if anything, Harley gives their dealers in extra incentives on carryover inventory, but if I were you I'd assume they get something. There is about $7300 in gross profit built in to the MSRP price of the bike, before any holdback or other incentives. I'd ask for at least $7000 off MSRP to help the dealer get rid of his old inventory. Then you need to determine how you want to sell the 2015 and for how much. If you trade it to the same dealer, handle it as a separate transaction from the purchase of the 2016 so you will know exactly how much you're getting for the trade and how much you're paying for the new one. You will definitely make out better selling it privately versus selling it or trading it to a dealer. Be prepared to be highly disappointed in how much that 2015 has depreciated.
JMHO - Jerry