We are about to move from a house with tiny bedrooms for my two ds's and decent space downstairs, to the opposite - big bedrooms each but more limited space downstairs. At the moment all toys are kept downstairs, but I think practically it makes sense to move them upstairs when we move.
But I am wondering how much they'll want to play upstairs anyway (ds1 is 5, ds2 is 2), and whether everything ends up downstairs. Also a big positive of the toys being downstairs is that they haven't really got a fixed notion of which toys belong to who - they are (generally) happy to share. I'd have to split the toys up between them if they were going into the bedrooms, and they may not share so readily.
How does upstairs v downstairs toy storage work for you?