I have been to both and in my opinion Metz is a much better destination than Nancy. The gorgeous Cathedral, the swans on the canal, the Temple Neuf, and one of the largest pedestrian zones in Europe. If you have enough time, you can also take the 45 minute TGV trip from Metz to Luxembourg City for a visit. BTW, the TGV from Strasbourg to Paris stops at the Lorraine TGV station, halfway between Metz and Nancy. You then take a 20-minute bus ride to either city.