Pivotal is an agile planning tool; JIRA is an issue tracking and classical project management tool. While each can be coerced to the other's function, it's really comparing apples and oranges. I'll admit that JIRA and Pivotal continue to muddy the differentiation with afterthought additions like JIRA's Greenhopper and Pivotal's time tracking. But JIRA is a good tool for issue tracking and Pivotal is a good tool for agile planning. If you're trying to use either tool for the other's purpose, you'll hate it.