Similac

Which Similac milk is the best for 6 mos old? My baby is exclusively breastfeeding and now that I have to work I need to make sure that she'll still get enough nutrients while Im away. I will still continue to give her my milk but just in case I ran out of stash she'll have something. My pedia suggested we should have Similac because all its products are premium. I forgot to ask which similac I should buy. Should I go for neosure?gain?pro advance?sensitive?. Any thoughts?? Thanks.