Black to play 5-3. So I made the brilliant blunder 15/7, of course thinking that if I got hit it would be game-over. Obviously this is terrible containment, and in retrospect I can probably even survive getting hit on the 15pt... and since breaking anchor is unthinkable, making the 3pt is the only remaining play.

My question is, what is ultimately the more important reason to play 8/3 6/3: making the 3pt, or keeping the "goalie" on the 15pt? As making the 3 point only reduces white's prime-clearing rolls by 2 (i.e. 62 no longer hops, but all other escape rolls still work), I'm wondering if perhaps the goalie factor is actually more important here since containment isn't super likely either way? Or is this still a basic case of "an inner board point is a permanent asset" blah blah blah?

well, in addition to making a blocking point 8/3 6/3 unstacks the the ugly stack on the 6 pt, which is a high priority i would say. 15/7 just creates another stack on the 7 point, which doesn't really help things.

also, it definitely is not game over if you get hit. white has a 3 point board with a blot, and you have the 20pt anchor so getting hit is not the end of the world

It's all three things you mention IMO.

The 15-point checker is an asset not a liability in this particular position, and you should be in no rush whatsoever to move it. It challenges the 11 point and covers the outfield if the runner hops. Meanwhile you have an anchor that is connected to the 15-point blot, and Villain's board isn't that strong.

Making the 3-point puts two builders to work. It's not great for containment at the moment but will get much stronger if Hero can make the 4 or 5 point.

Plus any home board point is a permanent asset. If Villain dances or enters awkwardly down the road, it will have done its job.

So I'm not sure what's more "important" but when all factors point the same way, it doesn't really matter.