Well, it's personal consideration then :), whatever suits you best.
As I've noted previously, I have a client who got offered a good chunk of change from Yahoo for his domain. He declined, waiting for the big bucks. Today he wouldn't get 1/10th of what the original offer.
And you may not be at the top of the serps forever. Three or four of these suitors decide to drop $500K over the next year or two, and your revenue can change quickly. There's something to be said for 'potential', there's also something to be said for the opposite - the revenue can dry up fast.
I had a guy call me years ago who bragged about his large house. He called it 'the house that Google built'. He made out like a bandit for a few years, top of the serps for a competitive industry. Then one day his rankings died and never came back. He'd put enough away that he wasn't too concerned. So there's always the 'bird in the hand' consideration as well. If you sell, and the rankings drop in 2 years, or 3, or 4, you've still got 7 figures in the bank :).