My experience come from android phones. I've used several 720p and 1080p phones for a while now. I currently own a 1440p phone.
I don't think that 720p or 1080p matter as much as ppi does. And it all really depends on what your using your phone for. Here are some things I found a difference in.
Text. If you read a lot of things on the Web you will notice that smaller text is a little more readable.
Icons. Some icons in apps like Facebook, Twitter, and forums will be a lot shaper and look a little better.
Images. High resolution images with a lot of detail will look a lot better. You'll be able to see detail without having to zoom in.
Games. I'm not much of a gamer on phones but that's another place were higher ppi will make a huge difference. Once games get support for the 6+ and it's 1080p screen, you'll start too see a difference between the 2 screens.
Of course it all depends on how good your eyesight is too. For most people, they won't ever notice the difference, which is fine. 326ppi is good enough.
Nexus ONE