One thing I think about also is if I will ever have an occasion to use same rifle unsuppressed . A 20" 300 win mag unsuppressed would be " tits on a boar" ..IMHOP...flame thrower maybe 
Hence what Karl Blanchard said .
Far from a guru just my 
You might be surprised. The muzzle flash is more a function of powder choice than barrel length. Some powders are just better than others in that regard. Here are a few examples that are a bit on the extreme end of the spectrum, but will give a pretty good representation.
20" VS 30" barrels on a 338 SnipeTac (150gr H50bmg powder). The shooting starts at 2:00.
338/375 Ruger in an 18" barrel (76.5gr H4350)
18" 300 RUM in an 18" barrel (101.2gr Retumbo)
I have more videos but they're all either shot in bright sunlight or in smaller chamberings (6mm BR, 260, 284)
I'll gladly take a shorter barrel over a little velocity. The extra length is the number one reason I don't mess around with suppressors.