Hey there,
Champix gets used more these days as it's newer, and has less side effects and contra-indications. It was also designed specifically as a Stop Smoking treatment, where as Zyban was originally an anti-depressant (people then noticed that they quit smoking while they were using it!)
Champix is a 12 week course of tabs, and you carry on smoking for the first week, but set a 'quit date' anywhere between your 8th and 14th day.
The most common side effects are nausea, sleep disturbances and weird dreams (!)
The doc might be reluctant to prescribe it if you have ever suffered from depression, or taken anti-depressants.
PM me if you need any more info
Hope this helps.
xx