SpringSource's goal is to make it as fast and easy as possible for people to build, run and manage Spring-based solutions. We have both Grails and Spring Roo because we deeply care about developer productivity and unquestionably both of these tools deliver a serious boost to what teams can achieve on top of Spring.
We have both technologies because Roo and Grails are very different at philosophical and implementation levels (as already noted in the other replies). Each technology approaches its primary language (Java or Groovy) and operating model (dev-time or runtime) with the philosophy of "how do we make the value proposition unbelievably good using this language and operating model combination?". As such you'll see each technology adopting a different style that maximises that combination (Roo's Java+Dev-time or Grail's Groovy+Runtime) and the commensurate benefits.
These differences are actually very positive, because they mean the Spring community can chose which "flavour" of productivity solution they prefer. While these initial differences around language choice and runtime/dev-time operation are immediately apparent, the choice of Grails or Roo also extends to more subtle considerations such as the default technologies used, user interaction model, IDE support, dependencies, standards, roadmap, extensions etc. Nearly all of these differences are a natural consequence of pursuing a best-of-breed solution for a particular language style.
Our best advice is to consider both solutions. Each have their sweet spots, but there are differences between the two which will make your overall experience better with one technology or the other in a given context. Both reference guides detail the respective benefits of each solution. Of course, remember the time investment is minimal in trying both out. In 10 minutes you can build a project in Roo or Grails, so give them a try and see what feels more natural for you given your specific background and project needs.
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