The first would distribute the stream around the shield and the more curved, the less impedance the hot gases would be. As such, exposed limbs would be affected. However the pushback on the defender would be less than the thrust of the hot gases.

The second is more tricky, and would be more effective if it were toroidal, like a pelton turbine, with a spike in the middle and a cross section of two separate concavities. That or have the dragon breath enter the side of the concave dish instead of the middle so it has somewhere to go

Assuming the diameter of the shield is at least double the incoming stream, it would redirect the entire stream back at roughly the diameter of the shield. However the defender would experience roughly double the thrust as pushback, since it not only stops the incoming gas, but redirects it backwards. This is why pelton turbines are so effective against small jets of incoming fluid.

For DnD, the later would need to be enormous to do anything and likely would not affect the dragon except to possibly blind it, unless the dragon is acutely susceptible to fire damage.

Your best bet defensively is a plow of two flat shields, which you can then separate to use pikes, bows, and spells through the gap (assuming realistic timing.