If you are making a game, I do not understand your choice of OpenSim and SecondLife. Depending on your budget, there are several choices out there, ranging from OpenSource to full blown Developement platform with high end graphics like BigWorlds.
By the way, OpenSim is a C# and not C++ if I am not mistaken. I am starting to look into it to develop several VW concepts I have in mind for a few companies I want to approach.