As promised I have now had a closer look at both my Mitchell 4575 kit and the Churchward Models 45XX that I bought from Precision last year.
Hopefully these photos will show some of the differences:
Churchward chassis - brass frame and nickel silver rods/slide bars - no detail overlay and different design for fitting cosmetic springs:
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Churchward body - brass throughout:
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The rolled parts are supplied pre-rolled in this kit - not sure whether that is an advantage or not as they still need further work!
Mitchell chassis - mainly nickel silver with detail overlay for the frame sides and alternative pony truck parts for narrow gauge:
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Mitchell body - brass throughout and with a couple of additional etches for later 4575 Class locos:
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This Mitchell photos aren't the full etches as I have started building this one but the this seems to cover the majority of the etched parts - there are a few detail items in a plastic bag that I haven't photographed.
It looks as though the Mitchell kit gives a wider range of options and for bunkers and buffer beams than the Churchward version but that is probably more relevant to the later locomotives rather than early ones.
The castings in the Mitchell kit are predominantly lost wax brass while the Churchward versions are white metal but they seem to be of a good quality and, frankly, most of them don't need to be brass.
Hopefully that is of some assistance. In practical terms I suspect that there is little to chose between the kits if you want a standard, 1930's to 1960's 45XX but for earlier locos the Mitchell kit, if you can get one, may allow earlier bunkers with less additional work.
The Churchyard Models chassis design has been updated over the years and this is different to the one that I built that sits under 4552 on St Merryn which I seem to remember was mainly a nickel silver etch, nevertheless this looks as though it will build up relatively easily.
Jeremy