Three things to consider when determining value to YOU as an individual.
1) desirability: the older smaller units tend to be more desirable in the overall market. In other words it is usually easier to find a buyer for one somewhere.
2) condition: better condition is of course worth more.
3) personal preference: what are YOUR plans for using it? Do you need to sleep more people (go bigger). Do you want easier towing with a smaller tow vehicle or easier parking in smaller camp sites (go smaller). That is strictly a personal choice.
How much time do you have to fix-up the 63 globetrotter? If time is limited that may be the deciding factor.
Obviously 3) can easily trump 1) or 2).
Most people decide what length they want/need first, and then look for availability, condition and price within a fairly narrow length range.