There's 3 revisions: 2.1, 4.0 and 4.1
I don't know much on the differences (I imagine most are bug fixes). I know the last version 4.1 is the one you would want for linking 2 Cruis'ns together.
The only thing I do know is in version 2.1, there is no Corn Pops advertisement billboard in Golden Gate Park as seen in the later versions.
I would be very interested to hear if anyone has any information to add though.