Which data centers for high availability

I need to build a redundant infrastructure for our web based solution in order to address availability needs. I cannot tell if two different New York data centers would be sufficiently isolated or would it be better to go San Francisco and New York? Would New York to New York latency be shorter than New York to San Francisco? I ask because either a reverse proxy to support sticky sessions or mirrored session data are going to require connections between data centers and both bandwidth and latency issues need to be considered.

The application I’m working with provides management services for service oriented non-profits, so we have a specific group of group of client using the service that are all located in the US.

Note: I selected the Load Balancing Tag because she same technologies are often used for load balancing, but my primary concern is continuous availability.