It's difficult to quantify in a Digital manner (0's or 1's).
But were all analog fans, so here goes.
The MMC20EN is a warmer, rounded sound, but still incredibly detailed.
The SMMC20CL+ is sharper, even more detailed.
For listening to stereo LP's, I like the MMC20EN very much, but that's my personal preference.
For CD4 LP's, the 20CL(+) series is necessary.
Anyway, IMHO, the best upgrade for this Type 55xx, is replacing all capacitors on the CD4 PCB (including Tantalums) with Wima MKS2 (for 0.22uF to 4.7uF) and Elna Silmic II for 10uF and above.
That will improve the sound tremendously, no matter what cartridge you have, at a much lower cost.
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!