Grin ... jiejie beat me to the 2 beaches comment this time. But seriously consider it, Krabi and Phuket are quite similar today. 20+ years ago during my first visit, it had a certain laid back fishing village charm. However over the years, it has become pretty much like Phuket. Between the 2, I would pick Krabi.

Apart from this, consider spending a little more time in Bangkok. Looks like you are only getting 2 days there. There is quite a lot to see and do. The markets, the river, the temples. Crowds are everywhere and it has a certain sense of urgency and energy. Check out the street food one night. Sit out late and have a couple Singha beers. I personally would spend perhaps 2 extra days in BKK and reduce the beach stay accordingly.

Cheers!