I've been using Scala for a little over a week now. Here are some preliminary thoughts on Scala vs Clojure.
(i) Scala is a better Java. Scala is still Java, in the sense that it is an OOP language requiring you to put a lot of effort into determining what is public and what is private. I've written about alternative approaches to OOP before [here, here, and here]. I don't want to go through the same discussion here (and don't claim to have fully grasped all the issues). I'll just say that I'm not smart enough to keep track of all the things you have to keep track of to do Java-style OOP. I can write pure functions that operate on data structures. I can't keep track of how all the data and methods and private etc. interact with one another. I couldn't do it when I tried Java and I can't do it now.
(ii) I miss macros. That surprised me because I don't write a lot of macros in Clojure. A lot of what I have done in Clojure makes use of Incanter, so it was just simple library calls. I don't even understand macros fully. Yet I miss them when they're not available. I spent a few minutes looking at Scala macros. That was enough.
(iii) Syntax is double-edged sword. I'm a fan of "everything's a list". If anything, I'd prefer to cut down on the amount of syntax that ends up in Clojure programs. Yet "everything's a list" just can't be sold to others. Scala, on the other hand, probably has the best syntax I've seen. I'd imagine that Scala makes a very good first impression. I haven't talked to anyone else about Scala so I don't know.
(iv) Scala performance is better than Clojure's. It's too easy to write slow code in Clojure. The various benchmarks I've seen support this. There are a lot of good things I can say about Incanter, but you wouldn't choose it for performance. I don't mean this in a way to start a flamewar, it's just an observation, and I'd be happy for someone to point out how I'm wrong. By that I don't mean a list of ways to tune my Clojure code. Tuning is a PITA that should be done by the author of the library. If performance is critical, I can't see a reasonable argument for using Clojure rather than Scala.
(v) I do mostly numerical programming. The JVM is a serious drawback for both languages. Beyond that, I don't see much interest from the Clojure community in numerical computing. All I see is Incanter, which doesn't seem to be very active, doesn't offer great performance, and is far from a complete solution relative to R, Matlab, and Scipy. Clojure is focused on other areas, and that's fine, but this is my comparison of Clojure and Scala, and it's what matters to me. On the other hand, Scala is very active in this area. Five years from now Scala will probably be a strong competitor to Matlab, while offering a much better language. It would be interesting to compare Clojure against SBCL for numerical computing.
(vi) Both have adequate IDE support in Eclipse. Counterclockwise is pretty good. The Scala IDE is just unbelievable. You don't need to debug Scala code: you just look at the bottom of the screen to see if there are any errors. You can even understand Scala error messages.
(vii) I probably have a preference for static typing overall, but it depends on what I'm doing and my mood.
(viii) Clojure has some rough edges. The transition to 1.3 coinciding with changes to Clojure/core left me frustrated. I use Counterclockwise with 1.2 and the old Clojure/core. I value my time. I don't think Clojure is where it needs to be in terms of documentation. Also, I really, really wish that those who write Clojure documentation would understand that NOT ALL CLOJURE USERS ARE CURRENTLY EMPLOYED AS ENTERPRISE JAVA DEVELOPERS! I tried Java years ago, hated it, and moved on. I know nothing about Java. It took me quite a while to understand how the hideous "com.Enterprise.Verbose.For.No.Reason" thing works when using libraries. I had to look in a Java book to get the necessary background. I have had a much smoother experience with Scala. Maybe the difference is that I've learned the Java approach by using Clojure.
(ix) Clojure has an impressive set of books available. They all do a decent job of explaining the language. Yet none of them can compete with Programming in Scala by Odersky, Spoon and Venners. You learn a lot more than Scala in that book. I'd highly recommend it to any intermediate programmer, even those who don't plan to use Scala. There are excellent resources available for either language so I don't see this as a reason to choose Scala.
Conclusion: I like the Clojure language better. I'd probably be more productive in Scala because it has a stronger future for numerical computing. I could sell others on Scala, but probably not on Clojure, due to syntax. Neither is going to replace my current combination of R + Fortran for everyday work.
That's a lot more than I planned to write. I'll update as things come to mind or as I learn I was wrong.