You will find that a number of aircraft are approved to takeoff with only leading edge slats extended, flaps zero. I dont know of any that are approved with only flaps extended. This would lead me to say that slats are more effective.
Wet V1 changes really depend on the aircraft date of manufacture. 4 engine aircraft usually only have a speed decrement, twin engined aircraft will have a speed and weight decrement. The V-speed decrements can be a lot higher than the 10 knots mentioned above.
Mutt.