Looking at the comparison data, the alarm led flashes are about half the amplitude via the clamp vs the resistor, showing the downside of the clamps lack of responsiveness, so the resistor looks both more accurate and having a faster response time. The clamps reading is too low and I'd trust the resistor reading, and this is our trouble that 20ma difference could be a good car from a bad car, ie 50ma and 70ma, if you can trust the reliability and repeatability of the kit (the clamp) you end up using an in line meter or doing something else to ensure confidence. So here I have no problem believing the resistor as better ...
The drift in the clamp readings is standard and due to the circuit design, the battery power reducing and also if you have the push button zero style, the capacitor discharging (so I've been told), which is why we need a long-term (2-4days) reliable method, and even the expensive alternative clamp suffers the same basic issues ... that's inherent in the clamp design.
On the door lock sequence it's amazing how the clamp is showing massively lower results in the peaks (about half), is this down to the clamps slower response times? I would like to see these validated against something else to know which is the true value, you might just manage to not blow the fuse on a 20amp meter! Sadly all the readings we've ever taken (over teh last 6 years) have been with a clamp, now I wonder about all those readings!
I would probably say the resistor (provided the maths and other stuff is setup correctly!) is always going to be a better method than a clamp, especially based on what I've read here and elsewhere on the web since starting the thread! Some way to valid this would be good, however a 3v drop in the circuit could itself start to affect things too (if using a 0.1ohm) so need to be cautious.
Clamps have their place, but have many "features" (issues!) that you need to understand to prevent yourself from making poor measurements. I guess I should re-read the response times etc and then re-compare.
Will place an order this week for some sample resistors both 0.1 and 0.01, maybe even 0.001 if I can find any at a reasonable price for measuring higher amp's? Any sources in the UK appreciated (Farnell / Rapid / RS ...)