I think it would depend on your end goals. Digital seems to be a more lucrative and marketable skill for the modern day but thats not a hard and fast rule either nor is a job or that kind of thing always a factor. If you do both, both will probably improve. I find that workign across art forms really helps my brain see things in different ways.
With digital too, like traditional, sometimes better tools help so look into brushes and palettes. Watch what other artists do, there is a learning curve.