Hairdresser every time. You can get some colour build up once you start to home dye, leaving you with different colours at the crown and roots and they don't always match up colour wise on the box (speaksfrombitterexperience). Hairdresser might suggest a combination of colours, not just one block which tends to show regrowth quicker. But it can get pricey so get them to recommend a home product for 'inbetween' appointments.
Do you already have permanent colour on your hair? If so you cannot lighten wothout the use of a gentle bleach, but you can add shades of warmth, red copper gold etc, brunnette shades from wella are fab, they just take away the flatness of a colour or the other alternative would be to have lowlights put in your hair, 2 shades lighter than your present colour, hope this helps...Well I have been hairdressing for over 25 years.lol