There is something to be said about "fun" as well -- Alliance could completely roll horde in one of the 3 other battlegrounds and get kill honor PLUS bonus honor... or they can suffer morale-degrading losses for 0 bonus honor and 1 hour games for a single AV token (keep in mind -- tokens can now be turned in for honor as well).

Which is more efficient?

Bonus honor matters, because you can get kill honor in ANY battleground. Therefore, the most efficient battleground is one where you can get both kill honor and bonus honor, and have fun doing it.