However, both JUnit and JMeter have its own limitation.
Unit test with JUnit is often single threaded unless you use write your test case with library such as GroboUtils.
JMeter is good to load test application that expose an interface to receive request (i.e. HTTP, JMS, LDAP, etc), but it is not easy to load test internal component of the system.
And it is good that since version 2.1.2, JMeter have introduce a new sampler for load test: JUnit sampler.
With the same set of JUnit test case, you may create multiple combination of test plan in JMeter easily via drag and drop. Then configure the size of the Thread Group to increase the load of the test. Run the test, and you are done.