This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
They're just different IDs for ActiveX controls - under the hood they're pretty much identical until you get into heavyweight XML stuff like XPath. So, on the whole, for Ajax development, it doesn't make a big difference.
The w3schools example is presumably trying both to ensure better support on a wider range of Windows-based web browsers. You'll see similar code in a lot of Ajax libraries, but you don't need to worry about it too much.
---<br />Author of...<br />'Ajax in Action' <a href="http://manning.com/crane" target="_blank" rel="nofollow">http://manning.com/crane</a><br />'Prototype & Scriptaculous in Action'<br /><a href="http://manning.com/crane3" target="_blank" rel="nofollow">http://manning.com/crane3</a><br />'Ajax in Practice'<br /><a href="http://manning.com/crane2" target="_blank" rel="nofollow">http://manning.com/crane2</a>