If it is real rice terraces you wish to see then forget all about those at Sapa and cross in to Yunnan Province of southern China via the VN/CN border cro0ssings at Lao Cai and Hekou and get to YUANYANG.
Yuanyang is at the same altitude at is Bac Ha and the same ethnic minorities as you will see at Bac Ha are also in large numbers in Yunnan province but with the addition of many more minorities people. The big difference between those in Yunnan and in Viet Nam is that those in Yunnan are all going about their daily work in their normal daily life and free of tourism.
Because of geography and geology, the cultivated rice terraces at Yuanyang and around are massive in comparison to those puny ones found in the Sa Pa region.
In fact, Yunnan province has such great diversity and history to offer tourists prepared to travel the large distances in China by express bus, train or plane. Yunnan province is a province of great contrasts and great beauty.
I've crossed this border quite a number of times but always from Yunnan in to Vietnam via Hekou/Lao Cai. Just be aware that China has a single time zone for all of China and when crossing between VN/CN there is a one hour time zone difference +/-.
The contrast between Yunnan Province and northern Vietnam could not be greater. VN is still in the stone age relative to Yunnan province, which is reputedly one of the poorer provinces of the PRC, with its magnificent express ways and railways etc which all stop dead at the VN border.
Sapa is really wonderful place for terrace rice paddy fields then. Beside that you can also choose Ha Giang is also very great for that. Ha Giang is lest touristy than Sapa.