I guess it really depends on which you prefer doing most. Both are equally attractive in their own respect. So the real matter is... would you enjoy one more that the other?? If the answer is no then perhaps asking yourself if of these two possibilities, is it possible to do them in another port of call. If you can say, do a beach day, at another port of call then perhaps you should consider doing the ruins tour, or if you are going to a port with better or more accessable ruins, then the answer might be to do the beach day on one of the cayes....Ambergris or St. georges are two really great cayes for lounging on the beach..
Personally since doing a beach day i would save it for another port. There is just too much to do in belize to waste it on a beach day. so many ruins... so little time. Heck just doing Xuantunich will take a whole port day alone. Same for Lamanai.. did the Altun Ha ruins last time, rented a vehicle and drove there my self. Between drive time and sightseeing it only took us 3 hrs. the other ruins are more remote, and take 1.5-3 hrs driving time and several hours of sightseeing. I think one would be pressed to see Lamanai in one visit due to the port constraints. being that belize is a tender port you have to curtail your shore time by about an hour, so if it says your docking time is 7hrs, you really only have 6... some ports should have longer docked times... belize is one of them...