Remember that the BTU rating for a grill is the total amount of heat output by all the burners per hour. This is typically measured not by the actual heat output of a grill but by the fuel consumption of the burners. Since propane has a BTU rating of 15,000 BTUs per pound a 30,000 BTU grill would consume 2 pounds per hour. The higher the BTUs the more frequently you will be getting your tank filled.
Gas grill manufacturers are aware that people tend to look at the BTU rating as a sign of power. They can advertise high numbers for a grill that just doesn't get hot and most large grills make a big deal of their BTU number. Manufacturers also try to make sure that their grill can reach a decent temperature. Of course they all have different opinions as to what that temperature should be. I suggest that you not put too much emphasis on the BTU rating for a grill and look instead at the quality of the construction and the reputation of the maker.