All above are true. I shot 5 different brands of rifled slugs through my 21" 870 smooth bore and found the Federal Tru-ball to be the most accurate.
As for the difference between a rifled barrel and a rifled choke. IMO I think a rifled barrel is more accurate than a smooth bore with a rifled choke. Especially with saboted slugs. I just can't imagine how about 3" of rifling (rifled choke) can initiate enough spin on a slug as it is leaving the barrel around 1100 fps.
I was contimplating getting a rifled choke. Let me know how you make out.