U'i is "good looking" and applies to people, Nani also means "beautiful" but is more applied to landscapes or inanimate objects.
i can't see no diference beetwen 'that beautiful woman' and 'that woman is beautiful'
For a simple sentence to describe a noun with an adjective, it would be Uʻi kēlā wahine. Beautiful (is) the woman. That is a complete thought. Otherwise, the adjective follows the noun 99% of the time. The words kēlā wahine u‘i just mean "that beautiful woman", which is just a phrase instead of a complete sentence.