Most manufacturers don't recommend shooting steel shot through full chokes. Modified provides a good pattern with steel without damaging your chokes.
Invector II's are a little pricey... (I have a newer Citori with Duratouch, 3 1/2 chambers and screw in Invector II's -- and just absolutely love the gun). But if you're going to have it tapped for screw-ins, see what the gunsmith recommends. If you don't have a problem with having a non-Browning choke screwed in to it, they may prefer to tap for Winchester/Mossberg thread pattern -- and those chokes run a little less $ as a rule.
I'd check with a competent gunsmith and see what they recommend.
Good luck and congrats for owning a very nice O/U.