If the principal fails and you have unsent log then it doesn't matter whether you go for automatic or manual failover, failing over manually won't magically make the unsent log records materialise on the mirror. Depending on the type of failure, you might be able to salvage the data after the principal comes back online. Of course, if it was a drive failure and the DB files are gone, then not so much.
Make sure the line can handle the expected load, test, test, test, make sure that the unsent log never gets above the allowable data loss for your system.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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