Brightening a dark video vs Darkening a bright video

Hi friends , I will have to buy an variable Nd filter for shooting in bright condition , they come in two ranges , 1-5 and 6-9 ,in very bright light 5 is not enough sometimes and if I buy 6-9 it is darker than I need from 7 onwards , so might have to go for either a bright look or a little dark footage than perfection sometimes, so several people on the net are searching for 3-7 range ... if I don't want to change my camera settings to brighten or darken , the question that just crossed my mind and I wanted you to share your views on it is .....

if you essentially have two choices , what would you go with in terms of color correction , darkening a bright footage or brightening a dark footage in post , which is more preferable ? , plz explain with it's pros and cons ... Thank you....