Lua is probably more useful in the long run, yes. But I'd say it's easier to implement the XML-system (assuming that you have a decent XML parser, like tinyXML).

There's a fair bit of things you have to consider when integrating a scripting engine; when to run the scripts, what scripts to run, what functions you should share with the scripting engine etc. You'd also have to learn the scripting language.

But yeah, go with the 3rd party scripting language. It's better in the long run.