DustDevil,
1) My preferred method is to begin tx at scene, stabilize, then move to ambulance, Why is because I get better results with my tx and it's the way I was taught.
2) My service has a SOP on max. scene times, 10 for Trauma, 20 for Medical, the SOP states we follow on majority of calls or document reasons why it took longer.
3) My Instructor taught us to do as I stated in #1.
4) Not everyone in my service does it the same, but all attempt to follow the 20 min time; Nor does everone in the 3 closest services to me do it the same, it is a controversy that is on-going in my area.