Here are the differences in specifications for the two models:
Rated Output Level
EPC-450C II - 5.0mv @ 5cm/sec @ 1kHz 4ma
EPC-451C - 3.0mv @ 5cm/sec @ 1kHz 4ma
Compliance
EPC-450C II - 10 X 10-6cm/dyne
EPC-451C - 8 X 10-6cm/dyne
Stylus Tracking Range
EPC-450C II - 1.5 - 2.5 gr.
EPC-451C - 2.0 - 3.0 gr.
Other specs. are the same and the specifications for the original EPC-450C are the same as for the II. They just redesigned the working elements from a vertical orientation to horizontal.
I have both cartridges with their correct, genuine styli and they sound very similar with the 450C II perhaps a little more "open" and detailed, possibly due to its higher compliance, but it's so close you probably wouldn't notice unless you were able to instantly switch back and forth between the two.
I track both at 2.5 grams. Neither have any problem with CD-4.
I have a stylus for a 450C but still have not found a body so I cant comment on its sound. Maybe some day.
I guess, if I had to assign "Holy Grail" to one of them, the EPC-450C II would be it.
Doug