Do you already have one? Pretty hard to beat the $20 rebuild kits for the stock ones, if it meets your needs. Rebuilding the alternator is one of the easiest things to do.
Nothing wrong with lifetime warranties, but after the first alternator rebuild, it will take you about as long to remove/install it as it does to rebuild it.
This is one of those items I like doing myself, and find it worthwhile due to (typically) cost savings.