Turkish air is not bad at all...again, it's not that hard to beat AA service. Personally I would take the JFK-IST direct with TK. I wouldn't worry about LHR either, just allow yourself enough time to connect to your flight. Are you taking a Norwegian Cruise? The reason why I ask is because my in-laws are coming on a NW cruise to IST on 4/10.
Well I would come one day early and enjoy Istanbul... it's a great city.

Cheers