Also, if you were choosing between just Berkshire and Investor AB for the lion's share of your portfolio today, based only on expected forward returns and level of confidence in obtaining them, which would you choose?

Hard to say. I can make a few comments, but I don't think they really address that question head on: it's just too hard.

I know Berkshire a lot better, so that would probably be the deciding factor, which isn't really a good reason. It does have a deep well of resilience, both philosophically and monetarily. That's probably the biggest advantage, and along with that the predictability that it brings. If I estimate the likely real returns over the next ten years, I'd be shocked to be off by more than 1.5%/year. That has a lot of value.

To whatever extent the past might indicate something about the future---
It looks like the returns from Investor AB have been better. 20%/year for a whole lot of starting dates this century, though that might be flattered by this being a stretch of fresh highs. One way to reduce that is to look at the slope of a trend line through the (log) prices: that comes out to just over 14%/year nominal since 2000 measured in euros, 15%/year measured in SEK. That could be thought of as a typical year, if the future resembles the past. It has been mostly one long bull run since the credit crunch lows.

Investor AB does seem to sell off in market turmoil more than Berkshire, and for a bit longer, which may matter to some people as a bad thing (don't like drawdowns) or as a good thing (buying opportunities). It really sold off hard in both the tech bust and the credit crunch, though not in the pandemic. You really want to buy it on its main exchange, which is in Swedish krona, as are the dividends. So that's a small nuisance.

Both Investor and its portfolio companies are fans of dividends. At the lower levels to flow funds upwards for reinvestment, and at the top level probably (if you scratch the surface) because the family likes income. Dividends for non-residents of Sweden are subject to 30% withholding tax. Generally, if you live in a country with a double tax treaty that rate is reduced and given a tax credit in your home country, so it's a paperwork hassle at worst. If there isn't a treaty, it's a dead loss, as for me.

I think perhaps Investor is a bit less dependent on ongoing large acquisitions to get good returns, compared to Berkshire? They seem to perhaps focus more on the development and growth of the portfolio companies they have. So to the extent that you think there might be a shortage of good acquisition opportunities at decent prices in the next few decades, that might be a point in favour of Investor?

Just curious — have you decided what the other one or two stocks would be?
Yup. Though of course that could change. At the moment my drop dead envelope suggestion for my widow includes 3 stocks and one fund.

Jim