Hi,
For diffirences of create data and normal data declaration pls check :
http://help.sap.com/saphelp_nw70/helpdata/en/16/0dce040cf711d3b9360000e8353423/content.htm
As you can see, with create data you can create dynamic variable, its about memory usage for more detail http://help.sap.com/saphelp_nw70ehp2/helpdata/en/88/84fb5269d34838a1f119b41dcdbc57/frameset.htm
Assigning data to field-symbols ( pointer of ABAP ) here is field-symbols help : http://help.sap.com/saphelp_nw04/helpdata/en/fc/eb3860358411d1829f0000e829fbfe/content.htm
here is general pointer logic :
http://en.wikipedia.org/wiki/Pointer_(computer_programming)
Now, difference betwen 1 and 2 is about dynamic and static memory usage.
Regards
Tolga