Quite a few differences...

The blue ( later ) versions have the 'deep' parameter configured later on in the circuit .This means there's more headroom ealier on in the signal path to deal with and more 'deep'.
Also the compression attenuation network ( optical ) is routed to be feed-back as opposed to feed-forward in the earlier versions....This gives a smoother compression curve and allows locking onto infinity with a lot more stability.
The blue face also has a wider compression range , at least another twenty db .
The input level toggle is configured for negative feedback .... whereas the 'white' face has that switch for pure attenuation.
The footbone is connected to the earlobe. :tu:

:w:

thanks for the info. Looking to get one used from a guy in NY, he has the older one.