As the author of Saxon I can't really give impartial advice... And I don't know the MAC environment, or TestXSLT. All these processors have a good reputation, though I think LibXSLT is generally preferred over Sablotron these days. If you're getting different behavior it might be a bug, or it might be that the spec allows different behavior, in which case its good to know that your stylesheet is doing something that's processor-dependent. So rather than just abandoning one processor, narrow down what the difference is and investigate it.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference