What do you prefer most while using MS-Excel, Mouse or Keyboard and Why??

If you know the shortcut keys Keyboard is always faster.

as for design issues, and formatting its easier to use the mouse, while for data issues of entry and editing then keyboard is much more efficient, this would be a skill gained by practice.

I will say both, it depend on what you are working on and if you know the key shortkuts or no.

Generally the keyboard is faster.