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You use COS when DSCP isn't supported on the device. "True" L2 switches are unable to "see" the L3 ToS containing ToS.
Most current Enterprise L2 switches are often "enhanced", and a common enhancement is they can "see" the L3 ToS.
Given a choice, use DSCP. (NB: it's not uncommon for some devices to use both. For example, many VoIP phones will mark a VLAN tagged frame with COS 5 while also marking the L3 packet with DSCP EF.)
