Hello William,
I think it depends on which "direction" you want to head from there. I think the difference between it and the four is pretty small but you're in a different world when you go from six to 8.375, unless you are using it as your primary hunting weapon; then I'd say you should take all the barrel you can get since that's the only gun you're carrying.
If however, you're talking about overall utility (home defense, targets, critters, cans, rotten tomatos from the garden), they're pretty close and if you have the extra cash to get a shiny 27 then you can split the difference with a five inch.
I just picked up a four inch 28 because it was a great deal, showed little use and decent care (I have to replace yet another extractor rod because some impatient nimrod buggered up the knurling as usual), and finally because I got it thinking it will be a conversion candidate and get a new barrel anyway. If a four and a six were side by side, barrel length would only be a tie breaker. All of the other things that we check out on used guns would be driving my decision. I care that little between the four and six.