Upvote:1
Cost of life is clearly cheaper in Leuven, not only for rents but also for food and leisure. If you aren't concerned by the time lost in transports, then you'll probably want to live in Leuven. You also have areas in Brussels that aren't too expensive like Molenbeek.
Upvote:2
In terms of money living in Leuven will be cheaper - a month ticket Leuven-Brussel will cost no more than 150β¬. I doubt that you can find such a low-priced room in Brussel.