Drink both Beers, with Pure Water you Absorb 70%, with a Diuretic (Coffee, Beer etc.) its 50%

Water Balance Article Caffeinated Beverages like Alcohol are Diuretics but they do not cause Dehydration, they just don't allow us to absorb as much Water as Pure Water. Beer also has at least 10% Carbos, so you will loose a little more to digestion, but get back some calories in exchange.

So Drink BOTH BEERS!