I would go with the small unit regardless of ratios, because if that is what I think it is ,
ie; the Nitro unit used under the 3cyl 497"motors in racing, then many good, used and proven hi-speed props
are available for it . Not that there are not props for the 4" unit, just not as many fast ones .
1:1 ratios are not all they are cracked up to be. Mercury gained performance with their 20H
back in the ’50s by going from 1:1s to 16:21s.
Go figure ?