SATA II or SAS in storage? FC+SATAII or SAS interface+SAS disk

I configure a storage disk array witch Fibre Channel interface 2 Gbps and HD SATA II 7200 rpm for cluster configuration 2 node with HBA interface 4 Gbps.
The adviser of my customer asserts insteand that SAS storage ( with non FC interface) and HD SAS 15000rpm is best performant for storage because 15000rpm is best 7200rpm.
I task am not therefore ! I think rpm sata II and 15000 rpm is only mechanic problem, instead the better difference is combination FC for interface with dedicated Storage Processor + best Cache and disck SATA II.What do you think about?