The only difference is that one machine has parity checking and the other
does not. RAM parity detects single-bit errors in RAM and prevents
operation with damaged data or instructions. This type of error checking
detects "soft errors" and intermittent "hard errors". Most soft errors are
caused by atmospheric conditions, but the reliability of current DRAM is
high enough that these conditions are VERY rare. Bad RAM chips are almost
always found during Macintosh startup without using parity checking.
Apple has never used ECC (Error Correction Code) schemes to detect errors in
System RAM. The Macintosh IIci and Macintosh IIfx (parity models) are the
first Macintosh computers to use any kind of memory error checking during
normal operation.
Apple implemented parity checking as an option for the Macintosh IIci and
Macintosh IIfx as a result of government requirements. Average users won't
need parity checking.
For more information, search under "Parity Checking".