Should i spend 3 nights in phuket and 3 nights in krabi or 4 nights in phuket and 2 nights in krabi?

i am goin to thailand in december for 9 nights so we spend 3 nights in bangkok and now we are in doubt over spending 3 nights in phuket and 3 nights in krabi or 4 nights in phuket and 2 in krabi or vice versa