There are two ways to reach the Spiti valley: one via Manali and Rohtang pass, the other via Kullu, Aut, and Jalori pass. If you haven’t seen the beautiful Kinnaur region, then taking the Jalori pass route would be a good idea. Even if you have seen Kinnaur, Jalori pass would still be a better option, in order to avoid the massive traffic chaos at Manali, caused by large numbers of trucks, tourists, and revellers trying to visit the Rohtang pass. A permit is required after crossing Manali to enter the Lahul-Spiti valley, if you go via the Rohtang pass, which is again a long wait in that chaos. So, in any way, head for the Jalori pass.