The GDDR5 is faster. In this case it's not just some different memory, but it really has an impact on performance:
http/www.gaminglaptopsjunky.com/gt-650m-gddr5-vs-ddr3-impact-gt-650m-2gb-1gb-memory-capacity/
The GT 650M is in such a level where is can really benefit from the bandwidth that the GDDR5 allows.