Brussels is bigger but a bit boring. It has a wonderful art gallery with lots of Bruegels and its central square is great, but really not too much else. Bruges is small and pretty, a nice town for wandering round. I can't help with hotels - Brussels will have the usual big chains, Bruges less so. I've been to both in the rain, andd both get rather grey and drab. For sightseeing generally I'd say Bruges; for indoor facilities, Brussels. If she's going on Eurostar to Brussels, remember she can continue on to Bruges at no extra cost, just show the ticket on the connecting train.