For fitness's sake, is it better to ride with low cadence in high gear or higher cadence in medium gear?
In terms of losing weight immediately, the obvious answer is to use whatever gear combo allows you to produce the greatest energy output.  For most people this will likely be a hair lower than the "preferred" range of about 70-90 RPM -- maybe 50-60, and pedaling as hard as you can.

However, if you want to KEEP losing weight by getting on the bike again tomorrow (or even just later this afternoon) then you should avoid muscle and joint injury by running at a higher RPM (70-90) and not being quite so extreme in terms of putting out effort.

And one should consider doing "intervals" of alternating high effort and low effort (eg, 5 minutes high, 10 lower), as studies have shown that this produces both better weight loss and better muscle building.