I would definitely recommend that you do the 3D/2N tour. Our group of 4 had a private transfer to and from Halong Bay and even then it's a long 4 hour trip to go for less time.
The food was yummy, the beer was cold and the staff was attentive. We were able to get out kayaking twice and to the beach for a little while for some swimming. My personal preference would have been to get to the beach earlier in the day as opposed to the 4pm ish time we went.
The dinner in the cave was a fantastic experience that I'm sure we will not forget.