The Waldorf Astoria is the best Hilton property in BKK, followed by the Conrad.

BKK-Samui by bus/ferry takes appr. 14 hours and by train/ferry appr. 17 hours. The train part of the train/ferry combination on a 1st/2nd class sleeper can be comfortable and interesting, but the waiting time, the bus transfer and the ferry aren‘t. I did it once and I don‘t want to miss that experience. But I won’t do it again.

If you arrive at BKK at 11pm, it’s a good time for party, but a bad time for connecting flights or busses. So you should book a hotel for that night. The only alternative is to book a private limousine to Samui departing the same night.

Bangkok is worth visiting for a lot of reasons. It offers higher quality and more choice than Samui. But at Samui you will find enough alternatives at an acceptable level of quality plus you can spend your time at the beach/sea. So if you don‘t know Bangkok well or at all, use that opportunity to visit it. Otherwise you don‘t need too.

Btw: If you want to go to Samui only, you should check xxx-SIN-USM as an alternative. Sometimes the award availability to SIN is better than to BKK.