RE: Remy 870
Barrel length will not matter, the advantage of the other barrel is it has interchangeable chokes. However I believe even with the choke removed there is still some constriction to it. Where as the slug barrel will be a true cylinder bore and should shoot foster slugs pretty well. Not to mention the sights are fixed to the barrel which should be more consistent.
The only thing a longer barrel on a shotgun does is effect the balance and the way it swings. Most modern loads burn all their powder in under 20 inches so the longer barrel will not yield you more velocity. Or least not enough that you will notice anyway.
Paul