Problem in Declaration - JavaScript

Hi,

I m facing improper display in the iframe pop up, while i am declare the Declaration 2, instead of Declaration1. What is the difference between the below two declarations:

Declaration 1
var frameid = '<%=frameToLoadMedHxPopUp.ClientID%>';
var dmidViewSummary = '<%=btnMedHxMainHidden.ClientID%>';

Declaration 2
var frameid = document.getElementById('<%=frameToLoadMedHxPopUp.ClientID%>');
var dmidViewSummary = document.getElementById('<%=btnMedHxMainHidden.ClientID%>');

siva