Both have their merits. Digital marketing is a broad field; it encompasses PPC, SEO, email marketing, and more. If you master DM, the earning potential is massive. But it's a steep learning curve and demands continuous upskilling.
Guest posting, on the other hand, can be lucrative if you've got solid writing chops and can pitch to high-tier sites. I remember landing a deal with a major blog a while back – the payout was sweet, but such gigs aren't everyday.