Rackspace is better (SLA-guaranteed reliability and 24x7x365 phone support), and cheaper ($2 per mailbox, no usage based fee).

There is an option for SLA with a paid Google Apps account. Last I checked it was $50/year, but comes with much larger inbox (25G at Google vs. 10G max at Rackspace). Also better integration and push notifications on mobile for Google, if you care about that.