In my experience parsing xml files with REXML was much slower than
reading from MySQL.
I had a storefront that read product data from XML that was dumped
from a legacy POS system. Page loads were SLOW.
I wound up writing an update script to sync the XML files with a MySQL
table and it became much faster. Not to mention the ease of use for
CRUD that comes with ActiveRecord.
That said, I could've probably just cached the XML (as you could too)
but in my case I wasn't writing to XML, just reading.
Also creating a model that houses the logic for CRUD/returning
attributes, would probably make you much happier, and your app more
rails like. that way if you ever decide to switch to db later, it will
be much easier.
But if you're building from scratch, I'd just go with MySQL (or other DB).