Creating iRule for web proxies with site preference

You seem to have made some good progress in a short time! :o)

So topology loadbalancing means that GTM will look at the source IP address and provide an answer based on that. For instance, if the source IP is in China, send user to the Hong Kong datacentre etc. Site preferences and fallback is pretty simple stuff for both GTM and LTM - as you say, with GTM you can set the loadbalancing method and have multiple pools ( i assume you mean using an explicit proxy at each site and use GTM to control the site/proxy in use ).

Otherwise, you can use a transparent proxy ( ie no browser configuration ) and use multiple routers as pool-members ( this can also be done with explicit proxy of course ). You turn off address and port translation on the virtual server and use a transparent monitor for the router. LTM has pretty good handling of failure scenarios like priority groups and fallback host. GTM has Last Resort Pools.

Also, managing source address matching is best done using a datagroup and the class match iRule command. Simple and scalable, you'll find plenty of examples on here.