I agree that a used Yaris is likely a better deal than the Mirage. I can only imagine the depreciation on a new Mirage. That being said, a lot of buyers want a warranty, and the Mirage has a damn good one. 5 year/60,000 bumper-to-bumper warranty, 10 year, 100,000 mile powertrain warranty, 7 year/100,000 mile anti-corrosion warranty. Out of a $12,000 car, that's damn good.
I'd be lying if I didn't say I'd thought about owning a Mirage. Great mileage, new car, low price, awesome warranty. There's a lot to like. Yes, it's basic, yes it's build quality isn't as good as the Yaris. Yes, the depreciation is terrible. But if you keep cars a long time, depreciation isn't an issue. No, it's not powerful, no it doesn't handle that great, but it's likely the handling can be remedied by the aftermarket.
If I were to buy a Mirage (which I likely won't; I likely won't buy anything to replace the Yaris unless it was totaled), I'd wait for the redesigned '17 model which looks a lot nicer.
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(Side note: I've had my Yaris for nearly nine years—the longest I've ever owned a car! Before that, my record was TWO YEARS!)