I want to just add that the food onboard TK is great... if you like Turkish cuisine. I'm not making a comment one way or another on that! But it does lean heavily in that direction. So depending on your palette, you may either really like it or not.

I would choose TK over DL A350, even with TK's old 777 hard product.

-Not as cramped as DL hard product for sleep.
-Personally, I prefer TK's soft product and food.
-TKs customer service isn't the typical US-consumer friendly one, so as long as you understand the cultural differences, it's just fine. I never had any issues with their phone reps either. The agents are there to help and will get things done; just sometimes without the smile.
-Try to find some layover time at IST if you can, and do a transit tour if possible. Don't forget, US citizens need an e-visa. You can get one on arrival, and should be around 30 USD (cash only).