This is a very personal choice and will vary by each user depending on their needs and uses.
ADVANTAGES
Low magnification: larger Field of View (FOV), larger exit pupil (more light to the eye), easier to hold steady.
High magnification: more detail in distant subjects, objects will appear closer.
DISADVANTAGES
Low magnification: less detail in distant subjects, objects will not appear as close.
High magnification: smaller Field of View (FOV), smaller exit pupil (less light to the eye), can be difficult to hold steady (hand shake will be magnified and more apparent).