Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

You might consider paying some attention to this recommendation. This guy
realizes that you will be knocking on his door *first* if you're unhappy with
the final combo.




Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the
new
one)

Also likely a well-founded recommendation. About agrees with your engine
supplier.



I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

If the original engine was 5 HP (about 20%) less than the new one, it wouldn't
seem unreasonable that your new setup can handle an extra inch of pitch.

You can worry a thing like this to death.
Most of the recommendations are pretty close, so if you want to stay with a 14"
prop the 9 vs 10-inch pitch won't make the difference between satisfactory
results and
anything dramatically less.