A 63 is pretty versatile and I know Sergio was running his Kyosho Spit with that motor in Silver and came in 2nd or 3rd in silver.  I would think the .91 is the best choice that you could use in all three classes.  I have one in my Dora and it runs fine for the Bronze the way I have it setup.  On the other extreme, in Gold Jason had that ME-109 last time and that thing did great in Gold, smaller airframe and different fuel mixture with the right prop kept that plane in contention the whole day.  For just one motor, rather than having a collection, I would go with the .91.