If you're ready to quit smoking, then you should definitely do so. But you'll want to plan carefully.
I quit a 2 pack a day habit back in 2008 when I was at my lowest weight and greatest level of fitness. In all honesty, the exercise is probably the number 1 thing that enabled me to succeed with quitting smoking. I channeled all my anxiety, cravings, freaked out-ness and etc into exercise and it really, really helped.
On the other hand, I almost instantly gained a few pounds, even though I was tracking my calories and doing everything properly. I had figured that i would probably gain 5-10 pounds while I quit and I thought that was fine because I would be able to lose it again once the worst of the quitting was over (say the first 6-8 weeks). I still think this is a sane approach. Quitting smoking was so overwhelming for me and so unpleasant that a little slack in other areas seemed a reasonable trade off for getting through it.
The trouble for me came when my life got disrupted and I stopped tracking my food and eventually stopped working out. I wound up gaining 70 pounds which I am now trying to get rid of.
I don't blame the smoking for my weight gain--clearly I made a series of really bad choices and succumbed to fat-person inertia--but it was a catalyst. If you can take anything from my experience I hope you will: 1. quit and use exercise as a tool in your process, 2. be reasonable about the effects of the first month or two of quitting on your diet--a gain of 5 or even 10 pounds is WORTH IT if you're giving up smoking, 3. STAY ON TRACK with your diet and exercise once the worst of the quitting has passed.
Good luck!!
p.s. 3 years later and I rarely think of cigarettes and am easily able to resist them at parties. Would that food and wine were so simple to resist! Also, it is GREAT to be able to run and breathe without being all phlegmy and disgusting.