If JRebel and OSGi are different, why do people compare them? Well, both JRebel and OSGi are capable of fast or instant class reloading during development; however, with JRebel this is the pure goal, whereas with OSGi this is more of a side-effect. In the end, using OSGi means switching to a brand new framework, while JRebel quickly plugs in to your existing stack.

http://zeroturnaround.com/blog/jrebel-vs-osgi-use-the-right-tool-for-the-right-job/

To put it bluntly, implementing OSGi should be done when the end goal is to achieve modularity and dynamism, but this requires continuous effort on your part; using JRebel is about increasing productivity by making Java as fast and iterative as Python, PHP or Ruby. It just requires download and pressing a couple of buttons in your IDE.

Perhaps you shouldn’t go for OSGi in a new project if all you expect to gain is increased productivity through quick code reloads. OSGi is made for projects that require a highly modular architecture. Regardless of your needs, JRebel can help you in either case.

For those curius about how the "JRebel vs OSGi" vame into being, this article links InfoQ interview with Neil Bartlett, and there they have that weird question:

InfoQ: What advantages are there of using OSGi over JRebel and an ordinary Java project?