"Better" can be meant in a number of senses. Contemplation is "better" in that it is 
 analogous to our final end, that is, contemplation of God Himself in beatitude. This is echoed in Augustine's "restless heart" where Augustine finds contemplative rest in "seeing" the Divine. However, I think Our Lord is here getting at something else. His words do not mean that the contemplative life is a "superior or "better" vocation or that those who choose a practical vocation are "lesser." Our vocation is ours in an intimate, personal sense. It is what is "better" for us to do with our own life! Marriage is the best vocation for one called to be married. Religious life is the best vocation for one called to religious life. Christ is here defending the contemplative life, Mary's vocation, from attack by those who would say "Do something useful with your life!" Its not about the superiority of the contemplative, but about defending it as an authentic way to live out the Christian life.