I would gather the costs to open the choke up as to threading the barrel wouldn;t be a lot different, But having the ability to change chokes is a benefit you won't get by just opening up chokes!
I am sure Briley can do either for you, why not email them and ask for a price quote on the work for each and see!
if you have any good skeet/trap ranges near you, you can also try contacting them as odds are they have either a smith on hand, or know of many close to you that might do work for less $$ than Briley might
as most smiths that are well know on trap/skeet ranges do a lot more shotgun work than the average smith does(IIMO< most basic smiths do more on rifles and handgun work as there seems to be more interest in them over shotguns)
so by going to either a well known shotgun smith like bBiley or one recommended from a shotgun shooting range will IMO get you the better results you will want !
either way, getting a price quote should be free to get!
before you contact them, I would suggest measuring the thickness of the barrels at bore and 4-5 inches down barrel,a s they will need this info to know if they have enough material to work with and be safe when done!