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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.
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