One reason you will see so many 20" octagon vs. 18" is that the 20" has been out for quite some time and has been the top choice of go-fast guys for a while. The 18" hasn't been out for nearly as long. Now, there is also 18 1/2" half-octagon (the brush popper) rifle. Not as many of the top guys use the round-barreled carbines although there are a few. The front sight on the carbine's barrel band is also not as easy (cheap) to change to a bead-type which most shooters like but there are options there as well.
So, yeah, try to go to a match and ask to handle some long guns. You might not find all the varieties that you are considering but you should get a better idea than you currently have.