Zone 1 or Zone 2 with 40mm LNB

Hi all,

Does anyone have any general guidance on the difference in performance effects between the LNBs at 37mm collar with an 'oval' face and LNBs with a 40mm collar and a 'round' face?
I do appreciate different brands etc.
The reason I ask is I currently have a 40mm and am wondering if its worth getting a 37mm collar LNB?
TIA.