At this moment, both are fine to use.
XML has that one benefit, it is easy to edit outside C2.
At a point in the nearly future (c3), Arrays will have their own 'value editor'. That will make things a LOT nicer. And bring the preference to using an array, i think.
If you choose for XML, there are a lot of tuts to help you.
There is also a template (when you make a new project, type XML in the search line)
If you choose for an array. Then specific for your question ....
You dont need the 'order', because the 'order' is already the same as the index on the X axis.
Far more easy is to start with an empty array, as in this example.
https://www.dropbox.com/s/lgad47eunduh6 ... .capx?dl=0
As you see, at this point is constructing the array tedious. If you have a lot to create, you are better of using XML.
But, that will change in the near future. And i am (personalty) very happy the new array editor.