Frankly speaking, when you have to decide on the old models and the new one, the Sony WH-1000XM6 has some valid improvements, particularly on noise cancellation and sound tuning. Sony increased the ANC technology to a new processor with additional mics, thus it is more adaptive in a real-world noisy environment such as aircraft or trains and reviewers are noticing that it is less selective than XM5 at blocking noise.
Battery is almost the same (about 30 hrs with ANC on) and comfort is approximately the same as long-session comfy, although not significantly different.
It is a significant improvement, therefore, over something older such as the XM4 or preceding. But in case you already have XM5s, it is a more discreet jump.