Decided to go with DL Biz- class for the reasons mentioned about luggage, intra Europe flight/connections and in hopes that current or future eruptions will not affect the direct route as the gateway city scenario might otherwise.
Would welcome thoughts on keeping up to date on fare sales on the JFK/IST route other than through Kayak and/or Airfare Watch Dog.
Thanks to you and the others who have posted and taken the time to resp.