I have two children, one of whom cares about my collection and one of whom does not. In my will, I am leaving the collection to them both to do with as they may. But I see now that this might be a problem if one wants to keep it largely intact as a memory of their dad and one wants to sell it. They could try and split it in half, but while many items can be priced via a price guide, some are harder to price because of lack of comps or because they are not cards.
On the other hand, leaving an investment portfolio is pretty simple. It is sold off and divided equally.
But I am not about to sell the collection to finance the portfolio (or vice versa)…even though lately it has been more fun looking at the collection rather than shrinking numbers on a screen

. Poor kids are just going to have to make do with inheriting my collection and whatever is left of my investments.
If he wants to buy and enjoy the card, then I am not sure where the investment advice comes into play. He should just buy and enjoy the card (it sounds like he is doing very well and this purchase would not impact him financially). And, as has been said, the more years of personal enjoyment he gets out of it, the more it becomes a personal item.
But I also don’t recommend the one card idea, especially if he wants to leave the children something they would want to keep as a memory rather than sell (if the idea is to sell it, then one card makes more sense, as does investment advice).
As has been mentioned, one card for two children could raise issues down the road. I like the two card idea, even if it may not be possible to get two cards exactly the same. Will the children quibble because they each have a similarly graded Mantle but the centering is not exactly the same? Maybe, but I think leaving one card to two people has more potential for problems.
Whether Mantle cards are a better investment than the S&P500, I will leave to others to discuss. I think it is amazing that this is actually a legitimate debate.
And the answer to “What Would You Do?” is to try and get into this guy’s will.