If I were to pick one boot out of your options, it would be the Cambridge. I have only owned the Coniston and it's a great boot, but I think the darker colour on the Cambridge would be more versatile. Not to state the obvious, but you should also consider which model fits you best.
I have been leaning toward the Cambridge, given that it's a great boot with lasted shoe trees included for under $400. With that said, I just don't think it looks quite as good as the Coniston (differences in last shape, eyelets, height, etc), but I agree that a darker color is a good idea (and the extra $100 for the Coniston isn't nothing either).
With that said, I just noticed that Purdey has the Coniston in dark brown. Stock is low but I emailed them to see if they're restocking any time soon. I'll report back what I hear.