I have flown both in the last month. LX from BOS to ZRH and TK from HKG to IST. Each has its own merits. It was my first on TK. I would take the time and connection convenience as the determining factor for my next trips. I am OK with trying TK and the lounge but would not make my decision strictly on these bases. TK plane was old and not maintained well but the mattress and the pillow for the seats was great. Food and service was good but being it overnight did not make much use of it. Lounge at IST is big but so what. It felt like an extension of the main hall with so many people. Tea was good. Why not get an isle seat on TK? It probably will cost you to make changes so that should be a consideration and you probably will enjoy either one being it your first on both.
Take LX.