At the end of this post I will tell you whether to use Rails or Grails for web development. But first, let me tell a tale of two decision makers:
How should Joe and Jane think differently about software platforms?
For my part, I am interested in Jane's problems. (And anyway, Joe often discovers he is actually Jane midway through projects.)
So how does this affect platform choice? If you are Joe, you care about specific details about what a toolset can do right now. Most of Graeme's Top 10 reasons are in the "Right here, right now" category. This is true regardless of whether you think he is right. (Sometimes he is, sometimes not.)
My advice to Joe: Know exactly what you need, and then pick the platform that comes closest to solving it out of the box. Depending on Joe's needs, either Rails or Grails might be appropriate (or neither!). A particular point in Grails' favor would be an established team of Spring ninjas.
If you are Jane, you care more about architecture. I mean this term in two senses:
If you are Jane, you care about how and why the platform was assembled, because you are likely to have to adapt it quite a bit.
Most of the commenters on my earlier post (and Graeme in his addendum) correctly identified the real architectural difference between Grails and Rails. Rails builds on Ruby, while Grails builds on Groovy and Spring. Rails wins this architecture bakeoff twice:
My advice to Jane: Rails over Grails.