You can drive to Spiti either from Manali or Shimla. From Manali, it takes roughly eight hours to reach Kaza and from Shimla you will reach in two days. It is best to do a complete circuit starting from Shimla, via Tabo, Kaza before finally ending up at Manali.
The best route for Spiti Valley depends on personal preference and road conditions. The Shimla route offers a smoother drive with scenic villages, while the Manali route has higher altitude and challenging terrain but offers stunning landscapes.