[Boromir88]

Legolas brings up a good point and it goes to the very beginning of this thread. The two different definitions of the word "Stewards."

Denethor's "Steward" is to rule and run the Kingdom as davem points out. Which was the job of the Stewards until a King should return. Hence the title "ruling Steward."

Where Gandalf's "steward," is to act more as an emissary of the Vala and Eru, and to be a caretaker. Also, I would like to point out Formendacil's point when Gandalf says:
"For I am a steward."
The keyword being "a," Gandalf is not the only "steward" there are others. And as Legolas shows Gandalf wants others to act as "stewards."

It comes down to the difference between the two definitions. Tolkien was a linguist, as he purposefully left one capitilized and the other not to emphasize the difference between "Steward" and "steward."