Buff (Colombian) Sussex aren't very common but there are folk keeping them locally to me. I could get some if I was more keen on "orange" chickens.
It's really the size/build of Brahmas that I like the look of though - I like big chickens! - plus everyone who keeps them seems to say they're generally very good natured birds.
My experience (3 dark brahmas, 1 buff, 1 light + 1 dark x gold penciled hamburg): pretty laid back. Dark have been willing for attentions from any roo. Both Colombian marked have been rather willing to rescue hens from unwanted cockerel idiots and have pecked or body checked the idiots. All my brahmas are more willing to nest on the ground than the other birds, but will take nest boxes too. The largest hens I currently have, and when they want a nest box, invariably, they choose the SMALLEST box in the coop. Within the hen ranking, they're middle/lower ranked, but among their peers (age groups), they're the top....maybe more due to size?
Belladonna in the tiny nest box
Rose in the favored ground spot
Primula
Rose on the lower bank of boxes. Upper occupied nest is the Belladonna nest.
The mix: has mother's mostly placid personality, but has father's alertness and small egg size. Has more mother's body shape, but less statuesque (brahmas remind me of the iconic opera singer, take that and add a bit of dainty oriental lady)
Shan (front)
Cheetah, hamburg father, bucket is a 1 gallon flat back for size ref.