Out of Linux gcc/g++, Microsoft Visual C++ in Windows, ModelSim, which is the best tool to stick to?
There is no such thing about these. If you are already used to Linux, keep using it. Or if you are happy to work with Visual C++, you should stick to it. The only advantage of ModelSim that may be apparent is that it is a complete simulation and debugging tool, and as such you may find everything in the same tool. DO your work in the environment where you feel comfortable.