I am structuring a trip with a long stopover (2+ weeks) at IST, and I will nest in some domestic TK flights on a separate ticket. The tickets have to be separate because my international itinerary is to be booked through work, and they won't allow me to put my personal-travel domestic flights on the business-travel ticket. My inbound flight will be TK82 from BOS in business class, arriving 16.00. The next flight on the separate ticket will be to HTY, and I have to decide between the 17.30 or 20.45 departures. I plan to have carry on bags only, and at the time of this flight I will have reached M&S Elite status. Which flight would you book?
Personally I'd book the later flight. Just a cursory look at TK82 over the last month or so on FR24 shows that it's late arriving as much as not. IST has a very large footprint and depending on arriving and departing terminal you'll possibly be in for quite a jaunt.
I'd take it easy, head to the business class lounge, have a nice shower, grab some good food and stroll to your next gate. Much less stress this way.