I think these sections may be what your looking for.
250.53(C) Bonding Jumper. The bonding jumper(s) used to connect the grounding electrodes together to form the grounding electrode system shall be installed in accordance with 250.64(A), (B), and (E), shall be sized in accordance with 250.66, and shall be connected in the manner specified in 250.70.
250.64(F) To Electrode(s). A grounding electrode conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system or to one or more grounding electrode(s) individually. The grounding electrode conductor shall be sized for the largest grounding electrode conductor required among all the electrodes connected to it.
Taken together IMO it is clear you may run GEC to the ground rod first but the GEC along with the bonding jumper to the building steel would need to be sized for the larger conductor required by either electrode.
[This message has been edited by iwire (edited 04-08-2005).]