on the fence vs undecided

on the fence and undecided both are adjectives.

Word	Noun	Adjective	Verb	Adverb
on the fence	No	Yes	No	No
undecided	No	Yes	No	No

As adjectives, on the fence and undecided are synonyms defined as:

on the fence (adjective)	undecided (adjective)
characterized by indecision	characterized by indecision
	not yet having made a commitment
	not brought to a conclusion; subject to further thought