I have had a 20 ga Superposed, solid rib, have a 12 ga BSS Grade II with 28" barrels and a 20 ga BSS Sidelock with 26" barrels. While the Super is a great gun, it was the only O/U I owned so I sold it to buy a Parker 16 ga VHE. Both of the BSS's I bought new in the mid 1980's. I had Pachmyer open the 12 ga to Mod/IC and it has been my Duck Gun for the last 23 years ago. It is a terrific Steel shot gun.
The 20 ga BSS Sidelock is a great upland game gun and I would highly recommend it today though they are pretty pricy. I didn't buy a 2O ga BSS Sporter at the time and often have wished I had one. Like a previous poster said, figure out your uses for the gun and what ever decision you make, they are all good guns.
Gordon