These two overlap on one job and diverge on everything around it.
Beacon is a lead-generation platform: it builds the magnet, hosts it,
and captures the leads, with plan limits on how many magnets and leads
you get. Unplain is a document tool: it turns your writing into a
publish-ready PDF with an automatic cover, contents, pagination and
branding, and leaves hosting and capture to whatever you already use.
So the honest framing is not "which is better" but "which layer do you
need". If you want the whole funnel in one tool, that is Beacon's
argument. If you have the funnel and want better documents coming out
of it, that is Unplain's.