I'd vote for Lua: faster and less heavy than Python...
Plus my biggest concern for Python would be how well the team can sandbox it. I know in this regard Lua also has advantages. Disclaimer: I am _way_ more proficient in Python than Lua, I just think Lua is a better choice.
I've recently been using Lua, and I love it. That said, I find python a far more appropriate language. Think about the use cases and the ecosystem around python.The quality, breadth, and depth of the Lua ecosystem for the finance/education/data users of excel will lead users to favor Python.
The main thing that might be nice is tables as a data structure for spreadsheets, but somehow I picture pain.
