A guardianship establishes a legal relationship between a child and an adult who isn't the child's parent, but it does not end the legal relationship between the child and the child's biological parents. For example, the biological parents are still legally required to provide financial support for the child. And if a biological parent dies without a will, the child has certain automatic inheritance rights.
On the other hand, an adoption permanently changes the relationship between the child and the adults involved. The adopting adults legally become the child's parents. The biological parent (if living) gives up all parental rights and obligations to the child, including the responsibility to pay child support. If a biological parent dies without a will, the child usually has no right to inherit.