AY’s advantage is their A350 every seat has aisle access. HEL is also a very easy transit airport. I enjoyed my SFO-HEL on their A330. Downside is 2.5 hours on anA320. Most EU business isn’t a great experience but it is comfortable still.

TK will or can depending on which flights you select be on all B777 which adds to the experience. TK’s lounge in IST is well known for a great transit experience. TK falls behind in that their B777s are 2x3x2 and they aren’t good in irrops.