I believe the tracker II comes with fixed sights and a stantard size barrel. Your are not supposed to be able to mount a scope on it, but I think some here have done it. The Tracker II is also lighter, so the recoil is worse. The upside is you can swap barrels in 20 and 12 guage.
The Ultra's come with heavy barrels and a weight in the stock. They are heavy, but better on recoil. The also come with a scope mount and no way to use open sights. The 12 guage Ultra use a SB2 frame which is for the 10 guage. The 12 gauge heavy barrels are 10 gauge blanks bored to 12 guage from what I understand. You CANNOT swap barrel on the Ultra 12 guages unless you want a 10 gauge bird barrel. The 20 and 12 guage standard barrels will not fit. Nor will a any of the rifle barrels. As far as having SB2 and putting the 12 gauge heavy barrel on it, I don't know? From what I undderstand the shotgun and centerfire recievers are not interchangable. I could be wrong about that though. I know for a fact you cannot put a centerfire on the shotgun reciever though.
The 12 guages have a 1:35 inch twist in both guns, so accuracy should be pretty much the same. You will just have to find a load it likes.
I would say if you want to be able to swap barrels get the Ultra in a 20 guage rifled barrel. This uses the 12/20 gauge reciever and you will be able to swap other shotgun barrels except the heavy barrel 12 gauge. Or a standard 12 gauge reciever and get the 20 gauge heavy barrel and fore arm.
Good luck,
Paul