I've started riding to get fit and lose weight. As part of that, I set little challenges for myself, eg: "I'm going to ride these next two blocks as quickly as possible in the highest gear". I'd be pedaling slowly and strenuously but moving quickly. As I got more fit I would spend by far the greater part of my 20klm daily ride in the highest gear.

However, someone at the bike shop where I get repairs mentioned I was doing it wrong - that I should be aiming for higher cadence and only use the top gears when going down hill.

What's better solely for losing weight?