I prefer data bound tags in the markup document.getElementById('<%#TextBox1.ClientID %>').value, over the use of the server side tag implementation <% = TextBox1.ClientID %>.
Server side tags prohibit you from adding controls to the dom in the code behind. This need commonly arises as you build out your application and the databound approach may save you from major rewrites.
When using server side tags also know as 'code blocks' performing this common operation
generates this error at run time:
The Controls collection cannot be modified because the control
contains code blocks (i.e. <% ... %>).
Unfortunately this often only become inherently obvious after you have built out your web site.
When implementing data bound control '<%#TextBox1.ClientID %>' resolve the value of control properties referenced in the markup, in the appropriate place such as the end of Page_Load data bind like this:
Keep in mind Page.DataBind() causes child controls on the page to also DataBind, this may be an unwanted side effect if the page handles the data binding of certain child controls separately. If this is the case, data binding can be performed on the individual control like this:
An applications evolution eventually leads to some sort of base site wide functionality where you may want to add base controls, once you've peppered you website application with server side tags replacing them with databinds becomes problematic, especially when pages have been coded to handle databinding on their own.