Ultimately the decision upon the year / era of rhodes you purchase is of course up to you and your individual taste. The earlier 1970 pianos have Raymac tines (aren't easily come by for replacement), possible felt hammer tips and different pickup wiring. These pianos have a distinctly different sound than later models. Pianos with wooden hammers and wooden side supports ran from 1971 to early 1975. 

1970 - early 1972 pianos employed a curve marcel pedestal action. This provides a snappier, more responsive action. Later 1972-1978 does not have this. For these later pianos, we can install a "miracle mod" to provide this.

Aluminum action rails, side supports and one piece plastic hammers arrived in 1975 and ran through the end of production. The visual aesthetic of the piano varied as well. 1970-1971 suitcases didn't have effects loops and the stage models did not have a faceplate until 1973.