DCS HMI S+ Operation architecture by ABB

Dear Friends
i need your expert opinion regarding the two optional architectures of the S+ system from ABB
1. Redundant S+ operation server and non redundant history server (Three server machine)
2. Redundant S+ server machines and merge history server as their part (Two server machines)
which option is less problematic and practical
while i personally believe that independent history server will make system simple and less problematic but redundancy of history server will be sacrificed
i will highly appreciate and welcome your suggestions on above issue
thanks & regards
Ashique Ali