While there are definitely some strong arguments for avoiding animal products and moving to a more plant based diet, having seen the first hand the distressingly harmful effects of smoking, if pushed to choose one thing to stop I would choose smoking.
If you could only see the actual lungs of a smoker as I have, you would understand what I mean. The tar alone turns the usually pink and viable spongy lungs into black hardened mucky tar-filled bags with worthless bulging emphysematous nonfunctioning bags of air rather than functioning lung tissue.
Its a site that's not easily forgotten.
Quit smoking, definitely!