Should I learn XML 1.0 or XML 1.1?

Unless you have a specific requirement to work with XML 1.1 (which is very rare), you should read the XML 1.0 Recommendation.

Tim Bray, one of the original editors of the specification, has created an excellent annotated version of it which you can find at http://www.xml.com/axml/axml.html

For a bit more info/opinion on XML 1.1, check out what Elliotte Rusty Harold has to say here:http://www.ibiblio.org/xml/books/effectivexml/chapters/03.html
Summary:

Everything you need to know about XML 1.1 can be summed up in two rules:

1. Don't use it.
2. (For experts only) If you speak Mongolian, Yi, Cambodian, Amharic, Dhivehi, Burmese or a very few other languages and you want to write your markup (not your text but your markup) in these languages, then you can set the version attribute of the XML declaration to 1.1. Otherwise, refer to rule 1.

XML 1.0 was first defined in 1998 and went through several (minor) revisions, but was never given a new version number. The latest one came out in 2008, as the 5th edition. "It is widely implemented and still recommended for general use."

XML 1.1 was introduced in 2004 and is currently in its 2nd edition published in 2006."XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features."

For more details, see:http://en.wikipedia.org/wiki/XML#Versions

The differences between them are very minor. If you are interested in that level of detail, you need to read both. If you aren't interested in that level of detail, you can read either.