SBS for serious HD or having fun?
if serious, then 14" is good and you can get factory SBS LE trade ins cheaper than a used one with 18" barrel, just keep an eye on the various classified boards.
for having fun, tax stamp for only 4" inch less seems like a waste of a stamp. Get an 870 with 4 round mag tube, cut back barrel to 12.5" after you get the form back.
if you want to be a super cool guy, get a Benelli M4 factory SBS, just be warned you will never be able to justify buying another HD shotgun because they will all pale in comparison.
you want to be the odd man out, get the Keltec SBS bullpup, either factory or form 1 the shorty they sell with the long barrel.
need a dog and halloween costume idea? get a side by side and chop it road warrior style.
these days with the shockwaves available and arm braces, not sure I would bother to SBS anything unless it was something just goofy short for range fun, see the Road Warrior special...