To ISO or Not to ISO

Schematron comes in two common flavors, Schemtron v1.5 (older) and ISO Schematron (newer). While there are a number of differences that can be read about on the official Schematron Website, it can sometimes be confusing about which version is "best". The following table presents some of the differences between the two:

Feature	v1.5	ISO
Varriables	Not supported	let element available.
Query Language	XSLT 1.0/XPath 1.0	XSLT 1.0/XPath 1.0, XSLT 2.0/XPath 2.0, EXSLT, STX, XSLT 1.1, etc.
Abstract Patterns & Inheritance	Not Supported	Supported
value-of Element (helpful in debugging and error messaging)	Not Supported	Supported
xsl:key Element	Supported	Not Supported (Workaround Exists)
flag Attribute	Not Supported	Supported
SVRL	Not Supported	Supported
include element	Not Supported	Supported

My suggestion would be to go with ISO Schematron, for the following reasons:

• Support for variables is important when working with ID and IDREF (more about this in a later blog).
• XQuery 2.0 functions provide a number of goodies that would be hard to pass up.
• ISO Schematron is a recognized ISO standard. . . which should count for something in a standards-based community.

Feel free to comment if you know any other key differences or disagree with my suggestion. Update: 2009-10-05 - Correct typo.