I need to study the effect of a particular mutation that is hypothesized to be tumorigenic. So which cell line should I choose to study its effect: a normal cell line or a cancer cell line?
What kind of effects are you expecting? Proliferation, migration, invasion, less apoptosis, a role in transformation? The choice of cells will depend on your output. If you suspect something increasing proliferation or migration, normal or primary cancer cells are good. If transformation is involved, normal cells would be better to use. And if you suspect a decrease in apoptosis, I thing cancer cells might be a good choice.
As you make the question it is unlike that you demonstrate any tumorigenic action of a mutation. It is not a matter of which cell but which approach (for instance how you "introduce" the mutation into a cell and what you will measure (as Christine points out)))