A mix of both almost to extremes which may need to both be balanced by yin and yang practices.
Writing can be very yin if you sit alone in a workspace and write. It can also be very yang if you follow classical theory and work out every movement and harmony according to it's standards, or generally think intensely while writing music.
Some physical activity (yang) would help for all the time being stationary, and some time away from writing, free and unstructured, could help to balance the mental intensity (comparatively yang) of writing.
This also gets balanced out with performance though, as it is usually quite exciting and then you have fun and drink (hopefully not [too] excessively) and socialize which is very unstructured.
Learning some qi gong and baguazhang basics would be one way to ensure that you can always add a bit more yin or yang without even leaving your work space if needed.
What a great response! Thanks so much, this is exactly the sort of info I was looking for. I do find that sometimes music can be over-stimulating; this can sap creative energies if it is not monitored carefully. It's a delicate balance because one obviously has to have feeling to write music, yet it is easy to get carried away and lose any objective interpretation of the piece or composition itself. I'll be looking into the practice you mentioned for sure.
I definitely do agree about the energy of live performances, whether one is performing or just attending. Since they tend to be very intense I think it is a good idea to precede this with more yin activity, which will be balanced out. A musician might optimize their performance by rehearsing for weeks prior but not playing at all the day before the performance - I've tried this and I was always able to return refreshed.