How do I get get-childitem to filter on multiple file types?

dir .\* -include ('*.xsl', '*.xslt') -recurse

I do not know why, but this one seems to be much faster:

dir .\ -recurse | where {$_.extension -in ".xsl",".xslt"}

The "filters" *.xsl and *.xslt can be part of the path parameter as well. Path can take a string array:

Set-Location c:\topLevel
gci *.xsl, *.xslt -Recurse