Interestingly, a lot of people choose fresh spinach believing it to be the richest in nutrients. However fresh spinach doesn't have a long lifespan and will have lost half its nutrients a week. Store-bought frozen spinach is frozen as soon as it's picked, locking those nutrients in, so in this case frozen is more nutritious. If you want to use spinach raw, such as in a salad, then fresh is better texture-wise. If you're cooking it though, it will come out about the same in terms of texture, so the choice is yours.