Excellent problem.

I would anchor on the 21pt, mainly because we are behind in the race and want to maximize contact. Also, we want to hold our front position as long as possible, and having our blot on the 18 pt seems more flexible for maintaining our timing advantage.

Interesting because I would have made the 18 point without thinking more than a second. (I haven’t plugged in this position yet to see if this is right). I looked at this as a prime-vs-prime position, and if we make the 18 point we totally shut down his ability to prime us, plus our timing is better, so I don’t see how we lose many games unless he rolls 66 and is able to out-race us. To me making the 21 point anchor looks like we’re doing him a favor by priming ourselves since that point doesn’t see daylight.

Of course, quiz factor is now making me think the 21 pt. But as far as I can tell all the signs are pointing toward the 18.

Agreed, I would make the 18-point without much thought. The 18-point checkers help control the outfield, while the checker on the 24 can move easily when he has to. This is really a mobility problem: after making the 18 our checkers have maximum freedom of movement, while after making the 21 we're still somewhat blocked.

Now that I’ve looked at the answer (21 point right by a ton), perhaps another advantage of staying back is that it stops White from closing his board and hitting a shot as an alternative plan for winning the game. He’s clearly losing prime-vs-prime, but if we leave he can try to make a 4- or 5- point board and hit a shot (which he’s likely to get) and our prime is just weak enough where he can probably escape his back checkers while we’re on the bar or our back checker is getting attacked. Keeping the 21 point reminds me of those pointy things cities put on park benches to stop homeless people from sleeping on them — it’s right in the middle of where he wants to lay his checkers down for his main winning game plan.

Of course I was totally confident in my initial (wrong) answer so take my opinion with a grain of salt

I don’t know if it is a good thing or not, but I was making the 21 point here with just as little thought as some were making the 18. The checkers blocking the 21 point anchor are going to be moving soon, so I am not sure mobility is a big issue. The big issue to me was game plans. Yes, the 18 is good for a prime vs prime game plan, but the 21 is not bad either. The difference to me is that white is not likely to want to cooperate with the prime vs prime plan as he has four uselessly buried checkers on his deep home points. To make use of these checkers, white would like to attack. He does not have any real immediate attacking threat, but making the 21 point will forestall any potential future attack making brown’s desired priming game much more viable.

Obviously I made the 18 here which is about a .11 error. But I'm glad I have company.

Here's a rollout.

1. Rollout¹ Bar/21 24/21 eq:+0.696
Player: 67.35% (G:17.85% B:0.74%)
Opponent: 32.65% (G:8.32% B:0.25%)
Confidence: ±0.016 (+0.680..+0.712) - [100.0%]
Duration: 2 minutes 11 seconds

2. Rollout¹ Bar/18 eq:+0.581 (-0.114)
Player: 65.68% (G:15.01% B:0.63%)
Opponent: 34.32% (G:10.34% B:0.26%)
Confidence: ±0.018 (+0.564..+0.599) - [0.0%]
Duration: 2 minutes 35 seconds

¹ 1296 Games rolled with Variance Reduction.
Moves: 3-ply, cube decisions: XG Roller

It's interesting that in this position there are two groups of us who both would make one play or the other without thinking much. I'm also in the 21 camp.

I agree the difference is interesting. Not directly related to this position, but over the past few months I have developed a little bit of an aversion to trying to make the 21 and 20 point anchors when there’s another good option available (because previously I was making a lot of mistakes splitting to them in the early game when it was too dangerous and I should have been running or playing 2 down or slotting instead). Of course they’re good anchors, and of course here we’re not splitting so the risk doesn’t apply, but I suspect that colored my instant choice to want to make the 18 instead.

I don't know which one I would have chosen in the end, but I would have hesitated a lot. I find these choices hard generally.