KMS does not required a license but with 6.5 you are limited to only 2 encrypted pools, 7.0 you can have 20 encrypted pools. KMS is easy to manage and easy to setup also and it is free. But you need to have atleast LTO4 tape drive and catridges.
Media server encryption option (MSEO) is a complicated one, lot of considerations etc... if it is a very large enterprise and dedicated team is there and dedicated infrastructure for MSEO is needed.
For KMS and MSEO backup window will increase, as you know.. because of the encryption.
I have used this KMS for the windows and oracle DB clusters in physical, in my previous organisation. But with virtualization its very huge data, and we need to modify the entire infra, we need to include the deduplication based arrays, since we have D2D2T architecture. So we dropped this for esx.
So KMS is less complex and easy to implement and manage,