Edited to reflect updates in the comments
Because it's a span, which behaves much like a div element, there's no inherent text or value attribute at play--it's a container, and you'll need to grab the value from inside the span container (see this answer and this article). Based on the fact you want the actual string value and not the html, the "textContent" attribute is probably the best bet.
You can use Selenium's "GetAttribute" method to get the "textContent" attribute of the span:
var textValue = element.GetAttribute("textContent");
// do code against textValue
There's no direct get accessor built into the Element class for attributes, so you'll need to pass in the string value for the name of the attribute you want, and it'll return the value if it's present.