Both sentences are technically correct but their context is a little different. ‘I will really stop smoking’ suggests that you are very determined about this decision while ‘I really will stop smoking’ suggests that maybe you’ve said you would stop in the past but didn’t. Now you’re telling someone that you really will stop at some point, probably.

I prefer the second one. It sounds a bit more natural.

I prefer the 2nd option since it sounds more natural to me. I would choose the 1st one if I wanted to add an additional emphasis to the sentence.