I collect both, and have always collected both (as an adult). I've got about 500 12" figures--Hasbro, 21C, all brands, and about an equal number of 4" 25th Ann and recent Joes.
I bemusedly note the segregation that many collectors put in place regarding both scales, and the denigration that some fling at the other.
Both scales have their charm and appeal, both have seen advances........both scales have their dumb stuff, and their black sheep.
I like the detail, and the articulation, and the cool-factor in the looks of certain figures.
I used to be big on vehicles, but space limitations have nixed collecting them. I don't collect the very-high end 12" stuff now, because the cost works against the fragility of the parts. I want something with a bit or durability to it, and a bit of "look"--and frankly the very modern "everything-is-Neptune-Spear" is boring as hell.
The price of some of these current figures is now greater than the cost of all-custom figures back when I started in the hobby.
Kitbashing is easier with the 12" scale, but no less fun in the 4" size--and the after-market products to enhance either scale are quite good.