As with hair, the same with nails.
Each persons nails are different. The shape, size and the persons coloring determines what to wear. When choosing nail polish or a type of manicure each of these have to be taken into consideration. While your nails may look really great with a french mani, mine might look better with full color.
The manicurist need to be well trained to do a good job of recommending what is best for your nails. Some persons can wear both and that case french might look better for work and certain functions.
Personally my manicure depends on my mood.