I guess you 1st need to decide how are you going to generate traffic. If it is through PPC for example, then definitely consider the high traffic niches.

But if it is through SEO, you may either be ready to wait for your site to get god SE rankings in high competition niches or if you want quick traffic, then low competitive nices could be your choice.

So, you need to 1st decide how are you going to generate traffic.