I have to say anyone who chooses one over the other is someone I want as a competitor because I will beat you every time.
Anyone dumb enough to do SEO without PPC is going to be easy to squash. And if you do PPC without SEO, then I will exploit that weakness and crush you just the same.
If you chose one over the other, and then couldn’t complete it in a competitive niche, it may have been me, or someone like me, that wasn’t foolish enough give a half-hearted effort at marketing, that kicked you out of the niche. If there is a niche that is highly lucrative, you will go up against serious competitors and your time and money will be wasted unless you step up your marketing and stop leaving half your business to your competitors.