Which way to build for iPad & iPhone better?

As title says, is it better to build for iPad first, then resize it down for iPhone version?
Or build iPhone version, then build iPad version from scratch with bigger graphics? I remember there's a program to resize a whole game directly somewhere. How are the results? Will the resized images end up blur?
Or build iPhone version, then build iPad version from scratch with bigger graphics? I remember there's a program to resize a whole game directly somewhere. How are the results? Will the resized images end up blur?
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