I work for SOASTA, so I'm well-suited to answer and will try to do so without giving you too much of a pitch. We focus on load and performance testing and deliver it both as an on-demand, full-service, pay as you go offering from the cloud, and as a self-service appliance for testing behind the firewall. We also provide functional testing, targeting rich Internet apps.
HP' suite of products are rather broad while we focus on what we do best: test websites and web apps, often in production, and at full scale. There are plenty of detailed differences between the products, but at a high level you might characterize us as a best of breed offering vs. a suite of (mostly) integrated products.