The difference between DCS and PLC is growing smaller. AB Control Logix IO cards are now rebadged and used as Honeywell DCS under an agreement between the two companies.
The BIG difference is SCADA for PLC doesnt contain any logic, it is still in the PLC. DCS generally has the graphics and the programming within one terminal so operators can view a logic chart, sequence table etc and fault find.
The advantage of PLC is that you can upload the program from the PLC if you lose your backup. DCS, lose your backup and your engineering station and you are in big trouble, no more changes!