Two things to consider:
1. Will you be wearing this in bed, as actual nightwear? Or is this more loungewear, i.e. what you'd wear around the house during evenings/lazy days?
2. How much do you move around in bed? Do you need nightwear to be tight so you don't get tangled up in it, or do you prefer looser clothing while you sleep?
For me, this would be loungewear, something I'd put on when
I got up, but not practical for me to wear in bed. I move around too much, and can overheat quite easily. I'd try both on, but probably go for the bigger size for comfort.