Only difference I am aware of is the tube, being an AC701k in the KM64, and a Nuvistor type 7586 in u64.
Sound is such a subjective thing, and i don't have any of the aforementioned mics to hand anymore . . . Not to mention, I hate describing sound differences remotely to someone I don't know! Some people use U87's and 414's for everything, and make amazing sounding records. I can't.
You might hate U64's. I doubt it, but you might. Especially if you like KM84. Sometimes KM84 was my first choice. All depends on what ya doin'!
From memory, there is a distinct difference in the bottom end response of the nuvistor and tube models, with the U64 being a tad thin, and undefined at the bottom. Top end has a lovely kind-of sheen to it. not the sweetness of the KM64. u64 is a lot more coloured than KM84, but in a very useable way. u64 with 10dB pad still sounds good, but KM84 can seem to change a lot with pad in.
Bottom line, they are all great. I would prefer to own all three, since they are good for different applications. Given a choice, and a limitless budget, I'd always prefer the KM64's overall, but if the price is right go for the u64's . . . just don't expect them to be as sell-able!
Also take note that whilst there are spare nuvistors available, finding a GOOD quiet one proved to be a bit of a chore! I had to buy a handful, and select. Having said that, I'd rather be trying to find spare 7586 than spare AC701k right now!
I am sure that there are those who disagree with me, but such is life. I do, afterall, make records for my living, so I hope there is some merit in what I say.