At the end of this month I can say I have got 2 alternative configurations that are working well.

My second Pixhawk did back from service and seems it works well.

I  test both platform at the same time.  Usualy Pi3 (art2) platform works as same as Pi0w (art2b).

There are small differences related modems but they are not important. Regular dial-in modems does not works well with PI0w mainboards due lack of power I think and this is reason why I use hi-link modem for PI0w.

Regular one is connected to PI3 but transfer looks the same as for Pi0w. Maybe my Hi-link model is less power consume than regular one. There is not more changes for those configurations.

Both Pixhawks are GPIO connected to Raspberry and I can say telemetry works very well.

There is not missing mavlink packets and Latency is also minimal.

Video streaming is different story. I still do not feel completely comfortable with Pi0w streaming possibilities. Maybe it`s only my imagination .
In next couple of days I will try to test PI0w somewhere outside building with LTE transfers. Right now I`m testing latency using Wi—Fi and I cannot find any differences.

Below you can check sample movie captured same time on one PC. Video pipeline is streamed at the same time form PI0W and PI3 as well.  Both are presented in different windows.

There is no differences between latency, it`s the same. Over video pipeline windows you can see also Raspberry desktops with power consumption meters in top right.

So what is the most important difference between those configuration.... On the screens you can see how many resources are consumed in the case of PI0w vs PI3. Pi0w consumes sometimes more than 50% of own resources and I hope it still enough for telemetry and video during flight. But this I can test only on reality and now everything I have now is only my expectations.