I don't see the Plus as being an iPad replacement, unless you only used your iPad for relatively limited things to begin with. Obviously, the iPad can run iPad software, which is often better and more feature rich than the iPhone versions, or at least better optimized to a large touch screen. Safari on the iPad defaults to desktop sites and displays them well. The iPhone defaults to mobile sites in most cases, and even when you force display of the desktop site it is not a great experience compared to the iPad.
But the other thing to consider is the difference in aspect ratio. If you're viewing widescreen movies, the Plus is nearly as good as the iPad mini.
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In fact, if you hold the Plus up to the iPad mini's screen, the letterboxed video on the iPad is only just a bit bigger than the video on the Plus.
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But if you're watching older TV shows in the 4:3 ratio, the iPad mini is considerably better.
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Also, I found the Plus to be much worse than the iPad mini for reading things like comics. The text is just too small on the Plus to be comfortable, so you have to view only one page at a time, and even then it doesn't seem as nice.
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I also use the iPad a lot for gaming, and the difference in screen size there is considerable. Basically, the iPad gives the better user experience in just about every case, but is obviously way less portable. So I generally use the iPad while at home and the iPhone everywhere else, or when I just want to check something quickly.
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I should probably add that if I was buying everything from scratch today, I would likely go with the iPad Air combined with the Plus, to make the difference in screen size a bit more meaningful.