LTM is primarily geared towards local load balancing via full proxy. GTM is primarily focused on Global DNS Load Balancing. When used in conjunction, The GTM receives the DNS requests, and forwards to the local datacenter's LTM depending on the method chosen (global availability, topology, etc. ) I would think that GTM standalone would be the your best option for the solution you are looking at. Once you decide that is the way you want to go, you then select the platform based on the expected load, factoring in for future growth.