Unless you run the U tube at a rate much slower than its rated for, 200gph on a 600gph overflow, for example there wont be air collecting at the top of the U tube. The flow rate is so fast thru the tube that air gets pushed along by the water. The overall flow rate might be the same on the C shell style but the speed that the water flows is slower because there is greater surface area for that water to move thru thus air has time to collect at the top.
For a snail to get into the U tube, and Im not saying it hasnt happend before, its quite an adventure. First the snail has to get over the oveflow fingers. This usually means venteruing out of the water for a period. Then the snail would have to make its way down the inside of the overflow box. And then find a spot on the back of the overflow box to hitch a ride on the U tube thats sitting inside the overflow box. They would then have to travel down the U tube and turn up inside the tube itself. Depending on how close the bottom of the tube is to the bottom of the overflow box the snails shell may physically not even fit between that space.
In the c shell style the back of the overflow box is part of the water channel that routes water up and over the back of the tank.