I personally don't like really small tablets since I have an iPhone, but I'd argue that the mini will probably be sell more in a couple years - but only because it's cheaper. And only when it gets a retina display.
If Apple were offering older retina iPads for cheaper than newer iPad minis, I'm pretty sure that people would invest in the iPad.
Most of us use our iPads as multi-media devices. I like medium size screens for my phone, but when it comes to a tablet, I think bigger screen = better. I'd rather browse web on a bigger screen. I'd rather view photos on a bigger screen. I'd rather watch movies and TV shows on a bigger screen. I'd rather FaceTime on a bigger screen.
I just don't get the portability argument - I mean can anyone honestly tell me that there are certain places one would bring an iPad mini but not an iPad? Both would be fine in portable areas: trains, buses, airplanes, school, etc. But both also wouldn't be good in non portable areas: wouldn't bring either to a bar, wouldn't bring either to a restaurant, wouldn't bring either to during a night out with friends, etc. - that's what our mobile devices (iPhone in my case) are for.
It's the same reason why you see a majority of Macbook users preferring Macbook 15'' over Macbook 13'', but at the same time, you see iPhone users saying that the GS III is too big, while the iPhone is perfect:
When it comes to multi-media devices that will more than likely require medium to large amounts of computing (ie. tablet, laptop, and desktop), then we prefer bigger devices. But when it comes to multi-media devices that will more than likely require small amounts of computing (ie. phone) bigger screens are not always better.
So again, I don't think that people will give up iPads for iPad minis in the future because of portability reasons, because iPads are more than portable and ideal for a tablet computer. But rather, I think it'll be more an issue of price.