PPC for me. I've been doing great with push notifications last year - did 6 figures. Never tried SEO though.
But generally it looks like this with PPC:
-- You need some budget to start with, but with some traffic sources like pops or push notifications, few hundred $$$ is enough for start
-- You see results within hours or even minutes, pretty much immediately
-- You have to keep running and optimising campaigns to earn, once you stop, your campaign will run maybe a day or two (well, it also depends on traffic method you use)
SEO (I'm not a SEO guy, so the info below might be slightly inaccurate):
-- Pretty much free, except the tools for SEO, however, you can group buy the whole set for $50 or so
-- You have to work for months if not longer
-- You don't have to work day in day out like with PPC