Good Question. From Wikipedia data, the R-1820 is heavier but has a smaller frontal area, which is expected with a second row of cylinders and roughly the same displacement. It probably is more complex with more cylinders, thus less reliable and higher maintainability (my thoughts). The F4F-4 and FM-1 used a 1830, but the FM-2 used a more powerful 1820 so there seems to be better growth in the 1830. Also, Wikipedia reported better specific fuel consumption in the 1830. It appears from my simple research that either engine would have worked by itself, so, why both? It seems commonality would have been advantageous. A look at the Navy, where I would think commonality is very important, I found this usage.
R-1820
SBD
F2A
R4D-2
J2F (Duck)
FM-1
R-1830
F4F
TBD
R4D-1
PBY
FM-2
Now the SBD and the F4F had to be assigned to same carrier. It sure seems an advantage to have both aircraft using the same engine. By the time these aircraft were developed, both of these engines were well proven, so backing up a design should not have been an issue.
I didn't look hard at the AAF, but they also are impacted.
Is it the supercharger profile?
Where are the engine experts?