I want to buy a device for testing my mobile web site in iOS, and I don't know what is better, getting and old iPhone 4 or iPhone 5 or iPad 3 or iPad mini. I only want to teste mobile web sites and maybe in future training in iOS development. I used online emulator but there's some difference sometime between real device and emulated that's what I want to buy one.
I'll say you should prioritise the iPhone purchase. From experience, website not mobile optimised tend to be easily usable on an iPad when compared with an iPhone.
Also, the behaviour of a webpage in iOS may differ from the Android. Keep that in mind.
Also, a page that looks good on an iPhone may not look good on an iPad. Take that into account as well.
And don't forget that safari isn't the only browser for iOS, there's Opera and Chrome as well (plus a few other not widely used).
Hope it helps...