For me both of them looks quite similar if it's going to features, but it's hard to say without using them (yet). So I have few questions:
1) Are they really feature comparable (more or less)?
2) Is there any example of enterprise or big open source system using any of them?
3) I have impression that Squeryl have better documentation, is lack of documentation in case of ScalaQuery real problem?
4) Which of them is growing faster and/or is faster on fixing the bugs?
5) Is any of them easier to use / more productive?