Townsend wrote:A bit more, both of the chimneys that already have caps are not the same in so far as which sides are open to draft; i.e. the single has an east/west opening while the large main has its south/north side open.
My first thought was that they did it that way due to the prevailing winds and potential for a downdraft...but your last comment shot that theory to pieces. I think the chimney will draft fine with either design, so the choice is yours.
Maybe I'm being picky, but if it were my decision I would try to match the original brick color and replicate the cap style on the other chimneys.