I'm saying that the $50 or whatever that Papers.app costs versus the $0 Mendeley costs is not a factor for many rational actors (if those exist ;-). The real cost is the time and focus to find and read the articles. This swamps the $50. The $50 should not be a factor.

If one or the other app happens to have features, tradition, or lock-in, in its favor, so that it will better turn your time and focus into increased subject mastery, that's what really matters.

I've found that having my papers organized by subject matter, searchable, and always available on any device, is a huge advantage in learning new stuff.

Over the years, it adds up, and you end up knowing more, which is helpful in lots of ways. To be crass -- it could be the difference between a 4% raise and a 5% raise for someone making $100K/y ($1K in the first year). That's a big deal.

Aim for journals with a high-impact factor. Researchers are encouraged to publish there. Unfortunately there's usually a pay wall for those. For free stuff you can check open-access journals. Some open journals also have a high impact factor and there's recently been a push for publish to publish only in open access journals.

Mendeley does not give you access to paywalled papers. You have to download them yourself.