Well for many of them there might be a long debate between Tellme and Siri, but in reality both are hardly even comparable. Both have nothing in common. Microsoft Tell me lets you dictate text and helps you to search on the basis of Voice commands, while Siri does more than that technically.
Microsoft’s Tellme is present in all the Windows Phone 7 right from the beginning and Siri is just the part of iPhone 4S. But functionally Siri does much more then what Tellme does and Siri is clearly the winner with the huge margin (really embarrassment for Microsoft ).
Guys at TechAU have done some detailed comparison. The video compares the basic voice commands.