Best load balancing mechanism to be used for a HTTP XML based service

We are planning on moving the Cisco Extension Mobility service to the LTM.

Basically the idea is to have four pool members, two as primary in DC1 and two pool members as secondary in DC2. The traffic is HTTP/XML based and will be used for the ip phones to login and logout. I would like to distribute traffic evenly between DC1 pool members and send to DC2 only if the DC1 pool members are unavailable. I’ve been reading up about Fast Layer 4 profiles to send only to the fastest node (DC1 members will be 1ms latency on ICMP vs DC2 being in the region of 7-10ms).

What options do I have?