Or go
with a 100 amp breaker so that it matches the one in the subpanel
(assuming fat wire for 100 amps)?

Choose this one. Leave the breaker there. If nothing else, it's a local
shutoff. Who knows you might need the addition juice some day, if not you
only have the additional cost of thicker wire.

I don't really need 100 amps. It also has a lot more positions than I
need (like double!).

yeah but.....

But at the borg, they have the little rinky dink
subpanel I have in the old house.

Do the 100.