The model 66 with a 3" uses a full length ejector rod vs. the shorter rod used in the 2.5" barrel. This helps with ejecting the empty cases.

I always found the balance of the 3" preferable to the 2.5" guns.

I only owned one 3" model 66 revolver. I bought it new in about 1990. My ex-wife ended up with it in our divorce last year.

I don't have the actual production figures, but the 3" guns weren't a standard production item and therefore there aren't nearly as many out there as the 2.5" guns.