I would say if you anticipate any sort of data science and machine learning potential in your application, then you should choose Django (Python) because you'll be in the home turf for this sort of work and you won't have to deal with language context switching... But this is a big if and may not be worth considering.
If your app has data of a large enough size for serious analysis, shouldn't you be building a data warehouse separately and performing analysis on that?
(scientific) Python is definitely better at stats out of the box than Ruby is though.
