Would you rather do Southeast Asia in December or the summer?

December for sure. It gets really hot and muggy during the summer. But it's also more expensive and packed with tourists during December. Thailand is especially bad.

I've done India during Monsoon (August) and Thailand/Cambodia in December/January.

The latter was definitely better weather but most of the time in India it was OK, though the first day there was the muggiest weather I've ever felt by multiples over LA.

I've booked Laos/Myanmar for October next year so here's hoping shoulder season works.