Hate to be picky, but barbeque is a food or a method of cooking. What you're looking at is a grill.
Anyhow...number of BTU's is not a good measure of a grill's cooking ability or even of it's capability for searing. The important things include number of burners (so you can do indirect cooking), ability to heat hot enough to sear food (600 degrees F) in a reasonable time, ability to maintain a relative low temperature (300 degrees F) over a long period without frequent adjustment, and the evenness of the temperature across the grill surface both at high and low temperatures. "Cook's Illustrated" and "Consumer Reports both tested gas grills recently and rached basically he same conclusions.
Tell us a specific brand and model, and you'll probably get more opinions than you want from present and former users.