what about Rxb7, Nxb7, then Ra8+?

Both sequences win.

maybe the problem should change so it only has one solutuion.

The 1.Rxb7 line does come out equall in points to the 1.Ra8 line. But after 1.Rxb7 the line might go something like 1...Kc8 2.Ra7 Rd8, if black relizes that he has already lost the major piece. And since you won the major piece already you want to go as far into the endgame as possible. And the 1.Ra8 line does take you further if black plays to avoid the pin after 1.Rxb7

1...Kc8 2.Ra8+ trading into the endgame.