Hi Vadhish. People in south dont usually celebrate Diwali. They celebrate Dusshera instead. So, if you're planning something for your Diwali holidays, I suggest you head to Himachal for a more authentic experience as they celebrate Diwali in a very ethnic way. Hope this helps!
Just for travel and sight seeing experience Himachal will be good in summers as well.If you plan to visit Pondicherry.. year end is best time to avoid humidity..
For enjoying lights of Diwali.. Himachal will be better as said by Pritha - Dusshera is more of celebration in south India than Diwali.
Diwali is celebrated normally in North India with common rituals and traditions. But in South India, you can explore entirely new ways of celebrating Diwali. They get up at 3:00 am to make preparations and worship and the traditions are entirely new. So, I guess South will be better if you have to explore the cultures.