I tried a Kindle Paperwhite at a bestbuy once, and although it's faster than previous Kindles, the page turn speed is still noticeably slower than on an iPad. Personally, that would drive me crazy.
That's an eInk thing-the refresh rate is slower.
OP, honestly, the Kindle Paperwhite is, as others said, a fantastic reading device. If you absolutely can only have one or the other though, I agree with the suggestions to stick with your old Kindle and grab a mini.
Alternately, by December we'll likely see new Kindles as well, and the current Paperwhite may slide in price. If not, there's always the basic $69 Kindle as well...so if you really love having a dedicated ereader, you can get the base Kindle and an iPad mini without stretching your budget to the extreme.
The major benefits to the Paperwhite, and why I reach for mine over my iPad or phone for reading, are the screen, the size, and the battery life. The screen is very paperlike, and the backlight isn't has harsh in the dark as an LCD, so it doesn't disturb my wife as I read. The size is perfect for one-handed reading, and it's light enough that you really don't notice it in a bag. Plus the battery life is quite good, one charge can easily be stretched out for a month or more.
On the other hand, it's just an ebook reader. That's all it does. So comparing it to a mini is a poor comparison, as the mini will always have more features and benefits, but they fill totally different consumer needs.