I have the 20" 590A1 I use it in 3 gun combat shoots and it preforms flawlessly with the ghost ring sights it shoots
without fail. It has very good balance for such a long shotgun. I have also put the GG&G sling mounts on it with a tac sling and is now the perfect combat shotgun. IMHO

 

Shotgun

I used to have a 20 inch pumpgun and I never had to use it in the house (thank god). But i did use it in some really thick brush (with the rifle sights it had it made a hell of a slug gun) and it worked great. I really can't see where the 1.5" difference would make much difference. Sure the pattern would be a little more open when it hits the guy at the end of the hall. But I am 6'3'' and 280 lbs and just as uncoordinated as I can be so the length of the gun probobly won't matter much. If I were you I would get whichever you feel most confortable with and which one meets your needs the best. If I really wanted the 18.5 and found a hell of a deal on the 20 I would take it and run. Just my 2 cents.

 

Subjectively - yes, it does make a difference.

It's the same reason why I prefer a 4 1/4" Commander sized 1911 over the 5" model. It's faster (for me) and I get no measurable decrease in accuracy using a slide that's 3/4" shorter.

I prefer 18" barrels on my tac shotguns as well for similar reasons.