I believe Unity to be a lightweight engine, it can display stuff nicely but is limited in how much simultaneously (correct me if I'm wrong). I am less tempted to go to a lightweight one, whereas OGRE is AFAIK much more solid and heavier and (for me) provides a higher step-over factor than Unity. 3DGS feels medium, but has the ability to rapidly get things working. OGRE's library requires some C++ project setup first in order to get things running. And that's quite a time consumer. But I have yet to try OGRE or Unity anyway, so who am I to judge.

For prototyping, 3DGS is pretty perfect, and for that reason I wont leave it.