after having done a little research on this subject, the 3.5 12 ga was introduced as close facsemily to the 10 ga.12 ga 3.5 loads can be bought with 1 5/8,1 7/8, and 2oz loads.while the 10 ga varys from 1 7/8oz -2 1/4oz.while patterns dont vary much between the 10 and the 12ga.the 12 is a bit more user friendly due to its being lighter, as well as a much wider use of applications and choices of ammo.[after all you would not want hunt dove and quail with a 10 ga.]
but i also found out that 3" loads can be had up to 1 7/8oz as well.
but just to set the record straight. heavy loads generally use less powder, not more.due to the fact that the heavy loads take less powder to create higher pressures. even this depends on the powder used.
source: Lymann 2009 shotgun shell master reloaders manuel.