How does a simple xml element unmarshal to a golang struct?

Assume the following xml element, with an attribute and a floating point value:

<thing prop="1">
  1.23
</thing>
<thing prop="2">
  4.56
</thing>

In order to unmarshal it, how should I define my struct?

type ThingElem struct {
    Prop  int   `xml:"prop,attr"`
    Value float // ???
}

type ThingWrapper struct {
    T ThingElem `xml:"thing"`
}

// VS

type ThingElem struct {
    XMLName xml.Name `xml:"thing"` // Do I even need this?
    Prop    int      `xml:"prop,attr"`
    Value   float    // ???
}

The usage of the XMLName Property confuses me. When should it be placed in the struct, and when in a wrapper as tag?