I have owned amps using both tubes, and even have some Mullard 6EU7s and all flavors of European made ECC83s stashed back. As far as a noticeable difference, I think the circuits made the difference.

For what it's worth, the two are electrically pretty similar. The gain is identical, and the plate resistance is very close. The 6EU7 has pretty low transconductance, lower than the 12AX7.

Thermionic