turbopixel :
That is very personal thing. That is no general rule that heavy mouse would make you better or worse in aiming.
Absolutely right.
OP, there are two main considerations when it comes to FPS aiming:
1. How comfortable the mouse feels.
2. Training yourself.
That second point is mostly about muscle memory. For example, can you keep the crosshair fixed on a small, specific location whilst moving around like you normally would? This kind of thing takes a lot of practice, but means that you can aim whilst keeping mobile.
A lot of people will tell you that a mouse with an adjustable DPi is important, but that's not true. If you train your muscle memory at 400 DPI and switch to 1600 DPI mid-game, your muscle memory goes out the window...