Difficult to choose. Ladakh as it has many attractions from breathtaking landscapes, the crystal clear skies, the highest mountain passes, thrilling adventure activities, Buddhist Monasteries and festivals.
Ladakh located between Karakoram ranges surrounded by Himalayas with Leh and Kargil nearby, it is a spot filled with abundant nature, cool weather and jaw dropping peaks. Summers are great in Ladakh Spiti on the other hand is known as trekking paradise which is completely surrounded by Himalayas. Summers pave way for trekking, mountain climbing and cool fresh air. I would suggest Ladakh though Spiti is also equally a lovely spot and must to visit.
We can't compare these majestic places. Both of them have its own uniqueness. Ladakh is more communicative aka you'll see a lot of people around but that's not the case of Spiti. Spiti's Atmosphere is more terrific, you can literally hear your heartbeat. But if you wanna explore wide then ladakh is your place.