why not go with both? That will give you a broad topic to discuss, chances are you will not run out of things to discuss. Just sayin.
I agree with Vitarank, but if you have to chose one or the other then SEO has a larger market understanding and would probably attract a larger audience. As far as competition goes, generally the more competition the better because it is a great sign that there is a market there. Usually if you find a niche with little or no competition it is because everyone else has stopped trying that niche because it was not working.