I use a a cheap Senco (Accuset) 16 gauge brad nailer for base molding
(3.5" high x .~5/8 thick) .shoots up to 2 1/2" brad.
To install base shoe (3/4 x 3/8 ) I use a Porter Cable BN200 that
shoots 18 gauge brads up to 2" long.
Any piece thinner than 1/2 could be at risk of being split by the 16
gauge brad nailer.
IMO the 18 gauge is way too small to use for base but is great for shoe
& other delicate trim.
I went through your questions about a year ago & decided I wanted both.
I even considered getting a 15 gauge finish nailer instead of the 16
but when I had the chance to get a 16 cheap I went that way.
I think the the relative "diameters" are as follows
18 gauge . 048"
16 gauge .063"
15 gauge .072"
so the big jump is from 18 to 16 gauge, I figured that 16 & 15 were
pretty much the same, plus the 15 gauge guns weren't cheap