I've thought long and hard about this today. Thanks for all the advice. I've spoken to the Omega AD and to the Patek AD. I'm going for the full gold speedmaster. This has been my original plan for a long time (years) and the Omega AD has worked very hard to secure this piece for me.
It will go with my 1969 speedmaster, making a lovely pair, and will also be a fitting way to mark my 50th birthday. I am going to wait and see about the stainless steel 50th anniversary model but I'm erring on the side of not purchasing this (I take the point made by someone made elsewhere that it is probably not needed from a collector's point of view given that this anniversary is covered by the other watch).
I'd like to honour my commitment to buy this piece from the Omega AD too. He has been brilliant at helping me secure a range of hard to get Rolex watches over the past few years and I don't think we'll be seeing many of the gold Speedmasters here in the UK either. He has shared with me in confidence some of the background to the acquisition of this watch from Omega and it was not at all straightforward. I am confident that in the medium to long term this watch will prove to be a good investment too.
Whilst I love the Patek 5205R, I think there will be a time to make significant changes to my small collection and get a complicated Patek. Just not now.
Thanks for all the advice. It has been greatly appreciated. I know many of you would have gone a different way with this nice problem and thanks to you all for sharing your reasoning here.