Unless you're going to reload for the 28ga ...ammo cost alone will push you toward the 20ga ...and you'll have a lot more used guns to choose from in a 20 ga than in a 28ga.
The older versions of the current Browning Citori Lightning .. in the late 80's it was just called a Citori ..and available in a 26" or 28" barrels. They have the older Invector screw in chokes..vs newer Invector Plus...but very solid guns.
They were available in both 20ga and 28ga...
20ga is still more versatile ...because you can go to 1 oz shells...down to 3/4 oz ...with the 7/8 oz being the standard 20ga load...
reloading for the 20ga - helps its versatility too.