A work guarantee is preferable to a UBI because work can and does give people a sense of purpose and meaning in life, identity, self-worth and social contact.
A UBI reduces the stigma of social welfare, because the payments are distributed to everyone in society.
Currently unpaid roles such as parenting and caring for loved ones/family, would continue to be unvalued with a work guarantee. The UBI would give carers this sense of purpose, self-worth and social contact that they deserve.
A UBI is preferable to a work guarantee because it allows people to define for themselves what they see as a sense of purpose, meaning in life, identity etc. whether that be work or something else which is most likely still useful.