As a person who has had the luxury to work in both ruby on rails and groovy grails, I've found a few differences that make their approach quite a bit different.
#1 Groovy allows you to write java. While this isn't a huge deal, it can be both a positive and a negative. I've worked on teams where folks treat grails as a super simple java framework and never leverage any of groovy's dynamic goodness. While this isn't a huge problem, it does delay (or eliminate) to transition from J2EE xml configuration file hell into a more dynamic way of coding.
#2 Ruby forces you to learn "the ruby way". For folks who are only used to java, seeing ruby code is like...seeing another language. Because of this, the idioms used in java are more quickly forgotten and you more quickly become a ruby native because you MUST. Only having worked with a few other people while they moved from java to ruby, I can only speak from my personal experience. I can say that ruby's syntax is not THAT much different as long as you keep an open mind, and I found I was able to more quickly learn the "ruby way" than I was able to learn the "groovy way" simply because I was FORCED to do it with ruby.
#3 Rails uses db migrations by default. This is a huge plus for db CRUD applications. It enables you to make sure you have a migration path from version to version of your code. Grails, on the other hand doesn't come with anything (by default) to handle this.
#4 Rails has a sparse model class definition. Should you NOT decide to use db migrations, you can simply create some tables in your database, create an empty ruby class, and begin using it. You don't need a class definition that matches the db table, because the fields are put on the class via introspection of the table. This then frees you to only implement business functionality on your model.
#5 Grails integrates seamlessly with most modern J2EE environments. Newer versions of spring allow you to code groovy code INSIDE your spring configuration xml. Grails creates a war file that can be deployed with little or no modification directly into a J2EE container. Rails CAN be integrated in a similar fashion, but it is really a kind of frankenstein implementation to get JRuby on rails via a J2EE container.
#6 Ruby on rails is MUCH more nimble and dynamic for building functionality. Grails enables taglibs and meta-programming, but many of the DSLs quickly get cluttered with java-like confusion that doesn't really have any business advantage. In addition, because of the way grails works with classloading in servlet containers, it is constantly restarting the container to pick up new functionality. With rails, I can reinitialize the database, drop/create tables, completely redesign the application, and it will typically continue to run without a hitch. I've often gone an entire day add/removing domain classes, changing controllers, rebuilding tag libraries and my rails engine never has to be restarted.
#7 The groovy and grails community is more organized. That having been said, they're both pretty disorganized and certainly the "herd of cat's" syndrome is running rampant in both of them. However, when I google "groovy roadmap", my first hit is this http://groovy.codehaus.org/Roadmap, "ruby roadmap" gets me: http://redmine.ruby-lang.org/projects/ruby-19/roadmap. You choose which one seems more organized.
#8 The ruby community is larger. While still pretty small compared to the likes of PHP or java, you cannot throw a stick and help but hit someone who has at least heard of ruby on rails. Groovy/Grails is still pretty small. On the other hand, I would point out that the grails community is growing where the rails community growth seems to have leveled off in the last year or so.
In conclusion, there are a lot of other factors that make selection of one or the other of these better or worse. If I where to learn only one of these and had no prior experience, I would probably learn ruby and rails just because of the size of the community. If I were a java person, I would likely start with groovy/grails just because the learning curve is going to be less steep.