ASP and XML are two different items.
Eventually you will have to know both when you start to get into some more advanced applications.
XML is more of a way to store and describe data - while ASP is a scripting language. XML is not a language in itself but more of a standard way to (store, describe, display) data.
In order to write dynamic XML for example you will have to know something like ASP.
ASP, PHP, Perl, JSP - are all basically the same.
XML is completly different - for Example HTML is basically like a really specific set of XML but is not that useful because it does not describe the data and is only used for display.
Anyways you'll have to know both so pick up your favorite scripting langauge (either ASP, PHP, JSP, etc - then you can use that with XML at the same time).
By only knowing one or the other (and not both) - you'll limit yourself.