Given that the watch requires an iPhone (which is basically a superset of the iPod touch functionality) they really aren't comparable products.
i.e., if you have a watch, you have what is, essentially an iPod touch (with cellular) to pair it with.
The iPod can't do stuff like measure your pulse, which is pretty useful for monitoring athletic progress. It's very encouraging to see real world results of your pulse rate vs. performance when it is tracked by the watch.
Also for resting pulse rate vs. diet / fitness. A watch regularly tracks your pulse rate, which gives you long term both active and resting trend information you simply won't get with an iPod or other "only used during workout" device.
e.g., for me with a change of diet (mostly quitting sugar/carbs) it was dramatic...
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