[anrakushi]

it is not a matter of which one is more common because they are used at different times for different things.

for example you would say 四月 "shi gatsu" for April but if you want to say 4 times you say 四回 "yon kai" and you would say 四時 "yo ji" for 4 o'clock (which is not yon, but yo)

九 "kyuu" is used for numbers, like when you say 9, 29 etc. but when you want to say time/month you use 九 "ku" like 九時 "ku ji" and 九月 "ku gatsu"

so it is not a matter of which one is more common, it is a matter of using the correct one in the correct situation. numbers and as an extension of that, counters, are difficult to learn at first but you get the hang of them eventually.