Adding some info:
As others have said, there is no practical difference (unless you're using DOS/Win 3.x that does not support 4 character extensions. This is basically the reason why file formats often have both 3 letter and 4 letter extensions).
Visual Studio defaults to .xslt.
On the other hand, the XSLT Specification (http://www.w3.org/TR/xslt20/) always uses .xsl except in one line where it says:
XSLT documents are most often identified with the extensions ".xsl" or ".xslt".
Kind of makes it hard to decide, doesn't it :)