I already said this in a comment but you asked me to post it as an answer, so here it is. :)
Rails won't really offer you much if you're building a rich client application. Client-side web development frameworks usually put most of the hard work on the client, and use the server only for storage and possibly some heavy computation (if needed). So I would personally argue that you don't even need Rails--you could go with something much more simple such as Sinatra. Since the client is the "meat" of your application, you will be doing most of your development there, so focus on finding a good client-side library/framework first, then worry about the server-side.
That said, I would try both and see which you like more. Cappuccino is very...different, and a lot of people are put off by it (mostly because of Objective-J I think). In my limited testing it also seemed to load much more slowly than other frameworks that I've used. I recommend you try writing a small application in it, and if you feel like it's not for you, then cross it off your list.
Personally, I would choose SproutCore hands down on this one, because you already know JavaScript (I'm assuming?) and the development style will be way more familiar to you. It will also allow you to use any server-side framework you want.
I don't know if you've looked at it, but there's also ExtJS, which is another very popular framework for creating rich web applications. I've used it and it's great, but the license requires you to either release your software as open source or buy a commercial license--I don't know about your situation, but this was a deal-breaker for me.
In the end I'd recommend you just try them. I can't tell you whether a framework will suit your personal taste.
Disclaimer: I have never seriously used SproutCore or Cappuccino for anything other than testing, so take everything I say with a grain of salt.