I'll share with you my personal experience about the difference between single link inserts and guest posts.
Guest posts: The thing is, many people think "the more expensive - the better" (that's not true always). By doing Guest Posts you will get a fresh article with avg. 3 OBLs; the truth is that those 3 links are not 3x better than the single link insert. Another crucial thing is that 3 links from one site cannot be the same power as 3 links from 3 different sites.
Single Link Inserts: According to my agency single inserts are most popular among SEOs, 'cuz, according to clients' words, those push sites better. However, that's not always the case. Another thing is that inserts are on avg. 30-50% cheaper than Guest posts (cost depends on supplier agency). Also, these links usually index faster and take less time to see results.
Conclusion: My recommendation is to try both services and see what works the best. There are some cases where guest posts work better than link inserts and vice versa. But, if you ask me what I prefer, then I'll go with inserts. The main reason why is because, according to my experience, those rank better and faster than guest posts. However, keep in mind that that is the case for me; for someone else, GP would work better.
Good luck with your ranking and cheers!