The two sonnets are related, and their themes are related, but they are not quite the same. Sonnet 75's theme is much as you say: the beloved will die, but will live forever in Spenser's writing, as will their love.
Sonnet 79 does say that the object of the poet's love will live on in his writing. However, it also draws a contrast between the beauty the world sees and the beauty the poet sees, claiming that the poet sees a truer, more spiritual beauty, and that this is what will last.
So, related, but not identical.
what is the personification in the poem sonnet75
what are the poetic elements in sonnet 75
what is the theme in sonnet 75