I have two RAID 10 arrays, one for the mail stores and one for the transaction logs. I have one drive left that can be a hot spare to either the mail stores or the transaction logs array, but not both. Would is be better to have the hot spare for one over the other?
Losing either will cause Exchange to shutdown, but in the long-term losing the database is probably the worst of the two to lose. I'd assign the hot-spare to the database (and would think about looking for a RAID controller that supports a global hot-spare... >smile<)