Just got back from Lamanai; we have not done Tuna Sandwich or Altun Ha. My impression was, while ascending the 2 temples about 9 stories was a trip highlight, the precious port time we spent in transit was awfully long, the boat ride was not any exotic Jungle Cruise expedition, and we would have seen about the same wildlife driving I-95. The San Gervasio ruins on Cozumel were a smaller version of the same thing. The hike and climbing the temples were hot and a bit strenuous (which we were okay with).
Lamanai was plenty worth doing--the question is the opportunity cost. Some other tour might be a better use of time, if you have never been. Xunantanich is probably not one, since on that one you sit in a van for 2 1/2 hours each way.