I'd say they are both equally dangerous but in different ways. Smoking is carcenogenic, even in small amounts, and contributes not only to lung cancer, but to a variety of other cancers. In addition it is implicated in Alzheimers and other forms of dementia as well as stroke and heart disease. Cigarettes are almost entirely responsible for COPD in older smokers or ex smokers. Nicotine is also a highly addictive substance and especially if people get hooked when young, the addiction can be extremely powerful and very hard to break.
On the other hand, smoking does not change one's mood or impair judgement like alcohol. You won't get arrested for smoking while driving while drunk drivers take many lives(often including their own) because of impaired judgement behind the wheel.
The best, healthiest bet for a long life is to neither smoke nor drink on a regular basis. I don't know how to choose between the two because both can be lethal and can lead to addiction. The key, as the ancient Greeks said, is moderation.
I've outlined the diseases linked to cigarettes, but I should point out that drinking alcohol can cause liver damage and lead to high blood pressure, gout, and heart disease among other things even if you are not a flat-out alcoholic. On the other hand one glass of wine with dinner or a Saturday night beer can actually be good for you.
If forced to choose, I would say that for the 15% of the population that becomes addicted to alcohol, it is more lethal than smoking. Those who drink heavily tend to be more highly addicted and die in larger numbers in accidents often taking others with them when they go. Also, many cancers are associated with heavy alcohol use and alcohol abuse always takes 5 to 7 years off people's lives
That said, for the rest of the population, I think the answer has to be cigarette smoking since a much higher proportion of people who smoke end up dying of cigarette related conditions than the people who drink die alcohol related deaths...since most people who drink do so moderately... as in an occasional beer after work, a glass of wine with a meal, or once or twice a year a night out on the town. The average smoker smokes every day and sucks on at least 10 cigarettes a day for many years and most smokers start in their teens. Studies show that even four of five cigarettes a day increase the risk of lung cancer, heart disease, and COPD significanatly.