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My answer is "it depends". I've been looking into them both as I was looking to go electric. I saw a while ago a real nerdy video analysing grind size/fines and extraction of a 54 vs 64 and the results were identical, so it seems it all comes down to the specifics of each...
64 is about 10 seconds faster, at least for espresso, and allows you to swap burrs and tinker if you so wish. The noise is said to be higher pitch than the 54 but will run for less time I guess.
54 is smaller, cheaper, takes 10 seconds longer and doesn't allow you to swap burrs.Because counter space is a bit of a premium for me, I don't fancy tinkering with burrs and am not in a rush to save 10 seconds, the 54 made the most sense for me. It's not very noisy either no louder than my Classic during extraction (I've ground with my 4 months old asleep in a carrier), while I heard people complain the 64 is very loud, though that's anecdotal/from the interweb. Hope this helps!
Anyone with experience of the DF54 and DF64 able to confirm the latter is definitely worth the 100 quid difference?