What are the main differences between EM 12c and Spotlight 10? Why choose Spotlight?
Main differences are price (Spotlight is less), a more intuitive interface focused on database diagnostics and Spotlight works across all editions not just Enterprise. There are many customers who use both OEM and Spotlight together as well.
Here is a link to a Tech Brief that explains how the Toad DBA Suite complements OEM and it has more detail on the differences between the two.