Like other Briley tubes, the Companions have a stainless steel chamber section and an alloy barrel. Typically, the light weight tubes use a lighter alloy for the barrel section and involve more machining. All captured extractor type tubes have a common weak point: the chamber wall is very thin in the 20-ga tubes. I had one well used 20-ga tube split under the extractor (this is not uncommon). Briley promptly replaced it at no cost.
Unless you're a comp shooter, I wouldn't recommend a pair of tubes. If you just want to be able to shoot 12 and 20-ga, look for a gun with two barrels. It's much less demanding than tubes. Remember, with tubes, to switch to 20-ga you must first meticulously clean the 12-ga barrels. It's much easier and faster with a simple barrel change. Also, the general shooting public is more familiar with two barrels on one action than tubed barrels.
The application of full length tube sets started with comp sheet shooters wanting a uniform gun for all four gauge events. You started with a comp 12-ga O/U and sent it off to have three pairs of tubes fitted to the gun. This worked fine -- with the weight of the tubes added, the 12-ga was smooth swinging and the kick reduced. To compensate for the lack of the tube mass when shooting in 12-ga events, a weight is often clamped to the barrel.
Further developments lead to tubes that didn't require custom fitting (Briley Companion) and the carrier barrel concept. With it you start with two matched 12-ga barrels, one is for shooting and the other is significantly lightened and is used with just the tubes, the carrier barrel. It plus the tubes is the same weight as the 12-ga shooting barrel. This is an expensive set-up and is for serious comp shooters who find the standard tube set up a little too heavy and/or don't want to worry about a barrel scrubbing when switching to tubes after a 12-ga event.