They are both great, it is a hard choice.
Kang has all outdoor tasks, and they are really funny animations. Marge doesn't have as many outdoor tasks, but the bonus of 50% payout for tasks...
Last year I bought Kang (I was in the same situation with limited donuts) but I bought Witch Marge this year.
Depends on your thoughts after Halloween is done. Last year (after Christmas for example) I hid the Christmas tree behind the Volcano Lair as it looked out of place, and if I had of bought the Santas grotto thing I would have stored it as it would look out of place any time other than Christmas. So depends if you will keep using the Witch skin majority of the time after Halloween is over.
Long story short I can't give you a recommendation either way, as they are both great, but hopefully I gave you some insight to help you make your mind up!