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how do they compare at accents?

Evi you have to be in the app, siri is easier to get to/use.
 
I find Evi understands my Glaswegian accent better than Siri, though Evi is just a glorified search tool, a very cool one, but still, that's really about it.

Siri is pretty great as a personal assistant, I've found myself using it a lot more than I expected.
 
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