The eventual redemption of the Lamanites will come through their repentance. The destruction of the Ammonihahites will come because of their refusal to repent. Alma very clearly sets up the condition of repentance as the key. It is not favoritism by God, but an equal condition upon Lamanite and Nephite. Indeed, the whole world must repent of their sins and come unto God with a broken heart.
In the admonition to the Ammonihahites there is an added subtext, however, and that is the idea that the Lord has higher expectations of those to whom he has revealed his gospel (in this case the Nephite religion which Ammonihah is rejecting). The scriptures synopsize this theme as “where much is given, much is required.”
Luke 12:48 But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.