An interesting thread - and it has answered a number of questions I was asking myself, the main one being: is the Professional worth the extra money? The short answer would appear to be no, particularly if you don't have the necessary equipment to enjoy the quality of 4k images at their full potential, which I don't.
As a consequence, I made up my mind and have just ordered the Advanced model. There were a number of other minor details that helped me to make up my mind, some of them found here, on this well informed site's blog, which is worth a read:
https://www.rcgeeks.co.uk/
One of the things I discovered there is that the Phantom 3's battery ( which isn't interchangeable) does apparently fit in some previous Phantom bodies but doesn't push all the way home. That being the case, the uninformed may still attempt to try it, but with potentially disastrous results.
I telephoned the company I have ordered from, just to confirm my order, and was told I was one of the first to pre order, so maybe the stampede to be an owner isn't developing as expected. They could give me no advice as to a delivery date so, having ordered, I now have to wait...and wait...