Mouse and keyboard all the way. Only exceptions being when I want to drive a vehicle with the controller as the steering is smoother and I have more control over my speed.
@OP: It would be very hard to hit the grenade button instead of crouch as the grenade button (if the game has one) would most likely be 'G'. Whereas the crouch button would is generally 'Ctrl' so you would be more likely to press sprint (Shift) than grenade. To the best of my knowledge I haven't played a game where you can just press a button and toss a grenade, generally speaking you have to select your grenade then 'fire' it.
@OP: It would be very hard to hit the grenade button instead of crouch as the grenade button (if the game has one) would most likely be 'G'. Whereas the crouch button would is generally 'Ctrl' so you would be more likely to press sprint (Shift) than grenade. To the best of my knowledge I haven't played a game where you can just press a button and toss a grenade, generally speaking you have to select your grenade then 'fire' it.