mostly vs more often than not

mostly and more often than not both are adverbs.

Word	Noun	Adjective	Verb	Adverb
mostly	No	No	No	Yes
more often than not	No	No	No	Yes

As adverbs, mostly and more often than not are synonyms defined as:

Other synonyms of mostly include by and large, generally.

mostly (adverb)	more often than not (adverb)
in large part; mainly or chiefly	usually; as a rule
usually; as a rule