George
You have seemingly decided to take offence when I am trying to help you.
I disagree that everyone else has been able to work out what you need'... ALL by themselves'. In fact I still don't know enough, though you have given some clues but it is still no where near enough. Do the same people know if you are going to be travelling abroad, walking the Penines or going to say Scotland , Wales or visiting the UK islands etc,etc. Do they know if you may have access to farmland, private set up hides or wish to visit the coast for waders or if you will have a desire or the resourse to create your own set ups for woodland and farmland birds. Do they know if you will be specialising in London's birds (there are several photographers who do)? Did they advise you that you might have some physical/practical restrictions on using large lenses at RSPB or WWT hides. May I have the crystal ball please
Some people have simply referred you to other sites for comparisons but I presumed by your original post that you have already done this and that you are struggling to make a choice. Nothing wrong with that...I'm in a similar position. Others have pointed out that there are considerations to be made in regard to weight and 'moments' (the ability for a force to twist or turn an object) , I prefer to use the term inertia but the point raised was valid and I have often raised the issue of inertia myself since it may have a bearing on say flight photography which is another area of specialty previously not discussed in your thread.
In your original post you stated :-
'I wander a lot and
ideally would like to be able to just about hand hold, often take photos of birds quite far away and end up cropping in aperture too.' (Bald italics for emphasis). I'm not sure what you mean by 'cropping in aperture too"?
In pane 4 you indicate sufficient cause for me to think that you would still like to just handhold and that the tripod was a bore to lug around. You stated that you use a tripod about 40% of the time. (This is with your current lens that has about 2 stops of image stabilisation. It is not therefore unreasonable for me to think that because the new lenses having 4 stops of image stabilisation that hand holding would feature prominently in your thoughts?). In post 14 you indicate a conditional decision to purchase the new 500f4 lens. There are a few folk out there who regularly use the older (and still excellent) 500f4 lens hand held and I know of a guy in the northwest who only uses his old heavy 600f4 lens hand held. When Andy Rouse used his Canon 500f4 he did so hand held and he is ,or was then only slightly built. Lenses are lighter now. Perhaps now you can understand why I am just a little confused as indeed you were on the 28th March indicated in pane 4?
In your reply to me you ask me what 4 f stops of IS means (nothing wrong with that but it is possibly central to the whole point of your decision making and I respectfully think that it is something that you should think about before buying lenses and tripods).
Whilst I appreciate that you have come to the forum asking in regard to the purchase of just one lens type I would urge you to consider whether just one lens is going to cover all your needs and I urge you to consider other camera and lens options. Like you, I live in a big city but possibly unlike you I have no car. When I was deciding on what lenses to get I had the option of getting an expensive Canon 500f4 and nothing else or getting a Sigma 500f4, Sigma 300f2.8 with converters together with a Canon 100-400.I chose the latter because it suited more purposes and conditions. I never regretted that decision. Like you I worked and had to consider a number of factors because of time limitations and limitations on light especially since I do a lot of urban birds and often in low light.
Folk have different desires regarding photography, they have different abilities, types of bird they want to photograph and have different styles, some are fortunate having bird rich locations they can get to and others not. Some are happy to photograph in in bright light and others want the saturated light that comes with the evening or the morning. Some want simply good record shots while others want artistic type shots. Some photographers start out casually photographing and then decide they want to step up to the mark of professional quality work. Some are desperate to get a shot everytime they leave the house some are patient. Without knowing your desires aspirations and ambitions, physical attributes , strengths weaknesses it is impossible to advise. We are all different.
I did not know whether you had any knowledge of birds which is why I used the word 'or' and not 'and' in the phrase where you have chosen to misquote me. I did read all the posts and I did so carefully, I hope. I thought long and hard second guessing what you might want or need but found it was impossible. My motivation was to assist you and save you from making a very expensive decision that you may one day come to regret. Entire books (or the major part of them) are written in order to help folk in their decision making and respectfully I don't think that you should make a decision based on a few replies from a thread on a forum especially without exploring all the alternatives and considering the future as well. In the end if you go for the 500f4 it may be that it is just the job for you...but your decision will not be an informed one.
There was no need for me to contact you privately save that I do not know what you mean when you say you can find no in depth discussion on bird's per se.
Whilst I agree with Pigeon Pete (Chas) that the Canon 300F2.8 with a 2X converter is likely a a sharp combination (MTF charts for the MkII lens and MkIII converter suggesting that the conbination is sharper than the 400 prime, so not to be sniffed at) and there is a great convenience for the combination, I have to disagree with his focal length evaluation. Focal 'reach' is a product of the real focal length of the lens and the pixel pitch of the camera's sensor. The cropped sensor just changes field of view (See Clarkvision.com). It is unfortunate that some leading camera manufactures pedal out this myth and it is also unfortunate that some leading photographers do so as well, even to this day.
I will reply to your question on what is meant by 4 f stops of Image Stabilisation in another pane later, that is, if someone else has not done so already.
Regards
Adrian