I like Scala, and I think Lift is very promising, but to answer the rhetorical question, I'd use Ruby because Rails is at a more mature stage in its lifetime than Lift is (I think Lift list just had its 0.2 release). There are some very promising numbers, and developing with Lift is certainly doable (although tutorials and docs are relatively scarce compared to other frameworks), but this may be a good time to harken back to PG's comments about technology firms who bet on the success of Java Applets (I mean, it kind of feels like Lift is a one man production, which means you're relying heavily upon someone who doesn't even know who you are...).

Scala is fantastic though, don't think I'm disagreeing.

For that matter, there are a dozens of other languages other than Ruby that would work just as well. Ruby just happens to have a decent framework for it, that happens to be well known.

Here is a link to wikipedia if you don't want to read the whole PDF: http://en.wikipedia.org/wiki/Scala_%28programming_language%29

Unlike Ruby, Scala is statically typed. However, due to local type inference, you can often omit variable type declarations.

Scala has the Lift Web Framework:

http://liftweb.net

Also see:

http://blog.lostlake.org/index.php?/archives/45-A-real-world-use-of-lift.html#extended