the reasoning to going ten speed is having slightly closer jumps between gears for the same cassette range, or a wider range with similar jumps -- if the rd can do it
-- both nicer for older knees

but yes, devil is in the details, as shimano stuff , cable pull, changed a lot around the 9, 10 change period, including the mtb rd's and shifters being very diff than road stuff -- so just be wary and do proper research if considering this route