The selection of virtualized network functions (VNFs) in service provisioning is a critical issue in inter-datacenter elastic optical networks. In this paper, considering load balancing of IT and spectrum resources, we propose two joint balancingfactors, i.e., Global Balancing factor (GB-factor) and Local Balancing factor (LB-factor) for VNFs selection. Then a Joint-Optimization Selection (JOS) algorithm is designed based on the joint balancingfactors. Simulation results show that GB-factor based JOS can achieve better performance compared with LB-factor based JOS and benchmark algorithm.