iRule vs load balancing methods on Big IP DNS Wide IP

Assuming you use the correct event, normally the target will be selected by the load balancing method and then you can over-ride it via the iRule (e.g. DNS_RESPONSE, or LB_SELECTED). Or you can use an event that fires BEFORE the selection and make the over-ride there (e.g. DNS_REQUEST)

It all comes down to where you need to do it, and what info you require. Normally however I'd fire on an LTM iRule attached to the GTM listener rather than writing a GTM iRule... Because LTM rules are usually richer (e.g. DNS_REQUEST is available for both GTM and LTM rules, but DNS_RESPONSE is only documented as being available for LTM iRules - Hmm... Wonder if that's a typo in the docs? - I'd test it for you, but won't be back and able to get to my test box before next week)

Note however, IIRC when using LTM iRules you don't get access to things like GTM pool member status...