Next on my agenda for comparing these two frameworks is package (aka dependency management). Up until the release of Rails3, I would say grails was the hand's down clear cut winner in this regard. Grails was engineered from VERY early on with the idea of dependency management being core to the framework. IMHO, this further illustrates how grails advanced the state of the art by sanding off some of the rough edges off of rails. If I were in charge of an IT department, I still think grails has a bit of an edge from a management perspective, but it does lose out a little in the flexibility department.
Ruby (via the gem mechnism) still suffers greatly from "gem hell" problems. Rails3 takes a step in the right direction by making bundler a core part of how applications are configured. Grails, on the other hand, is moving toward using maven as it's standard dependency management solution. In addition, grails has supported this for a number of years now and it works pretty well.
Where grails and maven suffer is that they are VERY opinionated. Unless you embed a jar directly in your project, it is very difficult to deviate from the "maven" way of packaging things. This means that building off a dev/snapshot package can be problematic (especially if you need to switch repositories often). Rails/Bundler on the other hand, while still pretty opinionated, let's you pick the method you want to pull your dependencies in. You can pull individual gems from git, some stuff from github, some stuff off your local machine. By embracing the culture of "everyone pitch in" they are offering the most flexible possible solution.
On the other hand, asking 3 people how to properly include a library in a rails project will likely net you 4-5 different answers. Grails, by contrast, will likely only get 2 answers...
Which of these two methods is better is left to you... I prefer the flexible approach of rails when developing software, but the more controlled mechanism of grails when maintaining software and infrastructure.