To be honest either place would tick the boxes for me but I would tend to lean on Chaweng Regent for the more Thai tropical lush atmosphere compared to the Resotel which is more modern and minimalist style.
The beach usually would press the win button for me at the Resotel but I like to feel like I am in Thailand and the resotel could be anywhere IMO.
As TM says read the reviews and see what everyone likes and dislikes. I don't take to much to heart about the reviews as some people are just so damn hard to please and will moan at anything and once again we all have different aspects of our holiday that are important.
Right now if I had the chance you could put me in any hotel in Samui and I would be happy. I go to Samui because I love the people the food and the beautiful feeling of the island rather than the hotel.
If you worry too much about your hotel you may take away the whole idea of a relaxing holiday.
You are going in the Monsoon season so you will need to go with an open mind on how your holiday turns out and just go with the flow.