The horseshoe-shaped tubes are easier to enter and exit, are lighter in weight, float higher on the surface and allow for the lifting of knees in shallow water. The new generation tube can also be inflated by mouth so you don't need to tote along a tire pump with your swim fins. All this sounds great but with new technology comes new problems. To create the open-face design, a bladder was required to replace the conventional truck tire tube.
Urethane is a popular material for constructing the bladders.
This is a synthetic material also used in many double-pontoon floating crafts we see today. While it's true urethane bladders are much lighter than rubber truck tire tubes, they are much more expensive, not as resistant to puncture and more susceptible to changes in temperature. The air inlet valves are both a blessing and a curse. You cannot fill the bladder with air from your local service station as there is no one-way automotive valve. Fortunately, you can build an adaptor that will allow you to fill a bladder with compressed air instead of relying on lung capacity.