It is primarily the width, length, they feel more open and spacious, the seats face forwards and you don’t have to put your feet in a small cubby hole.

I sleep on my side and I’ve flown this configuration on a number of airlines as well as Turkish (Latam, Air India, Air China) and I’ve consistently found that I sleep much better with this configuration.

That said, the seats are getting quite old now so they are not as new a shiny as the 787/350 and the PTV is higher definition on the 787/350. There is also more storage space for small items (phone / glasses…etc) on the 787/350. The 787/350 seats also feel more private - although that’s not an important factor for me.