The advantages of Leodis are that it's closer to campus and has higher quality rooms/kitchens/en suites. I wouldn't worry about it socially either, its definitely one of the more social halls. James Baillie is pretty similar, further out from uni but not too far, rooms etc are higher standard than average and is probably the most social. There isn't really too much to pick between them really and im sure you'd be perfectly happy in both. As far as I'm aware whilst both are over subscribed Leodis is (or at least has been) less popular so you probably have a better chance of getting it than JB.