Spiti in departing winter is also a good time to go. The temperatures are yet on lower side. However, the biggest disadvantage is you wont be able to go with from Manali side because Rohtang Pass opens in month of July-Aug -- So Hard Luck for You
Bdy I m there at Pooh location only in Himachal I will suggest u two things 1 If u want to do treekinng etc then it's fine to visit in March month 2 If u want to come on bike tour then it's better to come in mid June month as Kumzula paas will be closed before that and u will not be able to visit Chandraratal lake batal and complete ur route
If you plan to visit Spiti between March-May, you will have to take the Shimla-Spiti route, which makes the valley accessible throughout the year. This route passes from Shimla – Narkanda – Rampur – Kalpa - Nako-Giu Village - Tabo Village – Dhankar - Pin Valley (Mud Village) – Kaza