If at all possible I would recommend that you use a smaller provincial immigration office, in Trat a tourist visa extension takes around 10 minutes but in CW it could take a couple of hours depending on how many people are in the queue.
i have been to both but I am now on a long term visa so I haven't actually left Thailand for 2 years. My mate who lives in Bangkok says Burma is closer and easier but then again any border crossing is preferably to Aranyapratet Poipet border.
You have to go to your vet and get all the necessary vaccinations, check with the vet but some need to be done a month or so prior to departure. You will also need exportation documents from DEFRA and an import certificate from the department of agriculture in Thailand. Also you will need airline approved pet crates.
The vet will also have to give your cat a check up a few hours before departure.
It's not that difficult a process and DEFRA were quite helpful.
Once you land in Bangkok providing you have all the documents in order you will pay an import tax and take your cat home with you, no quarantine.