Hi, 
If you have just 1 or 2 days, Mai Chau would be a better choice because transfer to Mai chau just takes you 4 hours while transfer to Sapa takes you at least 7 hours by bus and one night by train. 
If you have more time, choose Sapa. Although both are beautiful with cool weather in the summer and so cold in the winter, Sapa has more spectacular views compared to Mai Chau. Mai Chau’s good for a rest, you’ll stay in a stilt house, try traditional foods of H’Mong people, watch they dance or go trekking. The view in Mai Chau are mainly fertile river valley with ethnic villige. 
At the same time, Sapa is mountainous area with spectacular terrace, high moutain views, many ethnic village to visit and the food are so good.
Mai chau

Sapa