In your case, you are doubtlessly much better off trading the cigarettes for ten or twenty pounds. Right now, get off of the cigarettes. You can deal with the weight later after you've beaten the cigarette addiction.
In the mean time, if you haven't already done so, switch to diet soft drinks; I know, the taste isn't as good, but the calorie savings is huge. If you haven't done so, switch from beer and wine to hard liquor without sweet mixers (better yet, cut out drinking entirely) because there's a lot of calories in beer. And step up on exercise a bit if you can.
But do trade the cigarettes for the weight.
In general, the question is impossible to answer because there are such extremes of each. Smokers vary from two or three cigarettes per day to two or three packs a day. And overweight varies from ten or twelve pounds to four or five hundred pounds. If someone asked which problem to attack first, smoking two cigarettes per day or being four hundred pounds overweight, my answer would be exactly the opposite.