I've done IAD->Europe both on TK and BA. I find the transit experience at IST much better than LHR: they just let us in to the airport by showing an onward boarding pass (no transit security unlike BA from a non CTA destination). I think the TK lounge at IST to be way better than anything at LHR since they have chef stations preparing the food in front of you for the Turkish specialities like Gozleme and Manti, and the Simit stand with local butter is a really great accompaniment to any meal. In terms of cabin service, the TK red wines are better than anything you get in BA business. I can recommend the Suvla Sur, but all four(!) selections are usually pretty good; I agree BA does better in spirit selection (my strategy for refills on TK is to ask during the service; they've always complied cheerfully, but I notice people who don't ask don't get refills). As long as you don't get a middle seat, I find even the 777 2-3-2 business to be fine, but the best they have is the 1-2-1 Turkoflot A350 which sometimes flies the IAD route and is a cut above Club Suites. In-flight catering is definitely better out of IST than IAD. TK has its own lounge at IAD, but you can also use the United Polaris one if you're willing to take the train from B to C (which has great cocktails and an OK dining experience).