Chris,
That depends on which configuration of 500 we're talking. I interpreted the OP to be asking between the 8+1 20" version and the 5+1 18" version. With the 20" 8+1 version you've got to swap the mag tube to be able to swap to other barrels. Not sure if they make a choked barrel for the 8+1 version in a 500. They do in the 590....
If we're talking about the 5+1, 20" version, I agree it is more versatile and the extra 1.5" of barrel length is not a hindrance. Awhile back Mossberg made a special run of 18.5" Heavy Wall accuchoked 500 barrels for a federal agency (Marshal's Service I think?). They sold the overruns to havlinsales.com and Havlin sold them to the public at large. I have one on my 500 now and it's a handy little joker. Throws evil buckshot patterns with an extended modified choke and Remington Express 2 3/4" shells. I swap out to the 28" barrel for birds and such, but the 18.5" does deer and turkey duty. I've got a 24" slug barrel for it that rarely gets used.
RR