The 10sp will work longer.
Question is, how much longer do you have to have it working?
Knowing that the only parts prone to wear and tear are the pulleys and if you're willing to replace them occasionally a RD of the quality of say Ultegra or DA will last for about twenty years, probably even more?
By that time I'm sure that group will be so outdated no one will even want to fit it on a new bike anyhow.....
I mean, why spend so much more on the DA for instance if the Ultegra offers the same performance and longevity bar perhaps a year or two extra?
Why fit the one specced for 10S if the 9S is the same thing?
If the cable fixing bolt is in a different place on various derailleurs, if the paralellogram is a different size or shape, etc. then a given amount of cable pull will affect the derailleur differently.
I agree but most Shimano RDs that are used for racing bikes do have the cable fixing in the same place and on the few older ones that don't you can easily reroute the cable and it will still work fine just the same.
Anyway, I just wanted to point out that you can often mix various parts of different groups from the same manufacturer provided you know how to go about it.
Much more so than even the more knowledgeable bikemechanics are willing to admit.
The manufacturer certainly is not going to put that in print for you, that for sure.....