Big decision to make!
If you want a better idea of what it's worth, contact Martin at McTear's. A few things to keep in mind if you want to sell it - do you know how it was stored all these years? Ideal storage includes keeping it out and away from sunlight, avoiding extremes of temperatures (cellar-like temps are ideal) and the Scotch should not have been in contact with the cork.
The decision is yours - I would likely have it appraised and go from there. But, if I couldn't account for how it was stored all these years, it may be something I would keep in memory of my grandfather and think of it as a reminder of all the great times we had together!
Whatever you decide - good luck!