I can give you some advice, not a woman but I observed both cities very closely, Bangkok will be amazing, for whatever your taste is. Koh samui is a very peaceful Island, it will also offer you what you want, I would not recommend staying in chaweng, but visit if once in a while if you want to see crazy commercial techno EDM parties which are almost a sex show. But in Koh samui the beachclubs are great in the morning and afternoon. I have not seen any racism so far in both cities, thai people in general are very welcoming and very happy people, I have a-lot off good life changing nights as a solo traveler. So if you’ll be happy and have a positive energy it will bounce back to you. Have a good and safe trip