[b-man]

Eleven cubic inches plus a better cylinder head and valvetrain design.

The availability of pistons that are reasonably priced is a big advantage for the 400 engines, good 389 pistons are scarce and expensive in comparison.

[Kenth]

1963 and earlier 389 blocks wont hold the starter.
1964-66 389´s may be bored out to 400 std.

400´s is still plentiful, 389´s is not.

[Sirrotica]

65 and 66 389 have thin wall cast blocks, plus the connecting rods were strengthened in 1967 over 1966 by adding a rib on the big end next to the rod bolt.

Then there's what Bart said about the heads, and valvetrain being improved also. Unless you needed to have a 389 for class rules, I would go for a 400, hands down.