Comparing XM3 to XM4, XM3 does seem to have better audio quality like I recall. I don't like what XM4 did with the mids, it doesn't sound as good as the XM3.
XM4 uses Mediatek chip vs the Qualcomm on the XM3. So, XM4 doesn't have AptX. But, people should do fine with AAC on both.
To me LDAC is almost useless. 990kbs is unstable. Connection drops.
I wish there was a way to save EQ on the XM3. Does XM3 have EQ memory that's keeps the setting for when you connect to other devices like the TV? It's a bit too warm for my taste and need to dial the low-end down.
I understand some people may prefer the QC35 for it's highs being more crisp sounding, but QC35's treble has an artificialness to it's sound I cannot stand. I do like the XM3 highs, although a bit dark. What I like is, it's quite silky, and not peaky or strong parts in it. QC35 and XM3 seems to on the opposite extremes, with 35 being a lean and XM3 warm. I usually don't like warm or thick, but in this case, my taste goes to XM3. QC35 sounds too unnatural to me. If we dial down the bass, XM3 gets better. That bass is the problem.
Anyway, I'm here mainly for ANC, and XM3 will be my goto for it. To me, it's the best ANC headphone if one does't care about call quality, multipoint and able to EQ the bass. Most useful for flights or scenarios when I have noise around me and need to watch or hear content. VERY PRACTICAL.