I think you are mistaking here.
I should be beam diameter instead of divergence.
a 1 W laser with 1mm diameter will look much brighter compared to a 1W laser with a 10mm diameter, No matter of the divergence.
When you measure a 1W with 1mm and 0,5 mRad to a 1W with 1 mm and 2 mRad there will be no difference (on short distance)
But with a higher divergence laser the beam diameter will increaser much quicker on distance, so the brightness will go down due to the thicker beam diameter.
To answer the question from the topic starter, I very often use my 1,5W RGB's together with my 4W RGB's. In smaller venues its hard to spot the difference.
But when you are fighting agains LED walls the 4W are a better competition to the LED wall.
Will the 12W have a WOW effect against the 3W, No, they only will be a bit brighter. If you planning to do small (lets say less than 1500 people) shows better invest in more 3 W projectors as 8 x 3W will have more WOW effect then 2 x 12W.
But if you are planning to do big shows (with allot of conventional lights and ledwalls), or Festivals, definitely invest in +12W projectors is my opinion.