I've seen both and they are noticably different. Why do you need to lie about a phone display?
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Easy for me discern the sharpness. Maybe you didn't look closely enough?
If you hold it at arm's length, you're right, there's no difference. If you look closer, the sharpness on the 6+ is breathtaking.
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Yes, it is easy to see.
330ppi isn't enough. On my 4S I can easily see the pixels. The goal is for the pixels to be invisible.
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Actually, content on the 6+ is slightly smaller than on the 6 due to the downscaling. Not very noticable and since the 6+ can display finer detail, it's no problem to read smaller text.
I think you're confusing resolution with pixel density. If two devices use the same pixel density and no funky downscaling, then all else being equal, the larger device will simply display more area. Text will be the same size. Perhaps you're thinking of two displays of the same resolution, but of different physical dimensions? In such a case the physically smaller screen will display finer detail.