How is the iPod Touch or an iPad Mini a replacement for an iPhone? Have you priced data only plans vs. iPhone plans? iPhone plans are far cheaper than data only ones and also offer phone, texts, more data and subsidies with also a lower price. Good luck with an iPod touch if you need to call 911. Like the other poster said I want the largest device I can carry with me and still be portable and I certainly do not want yet another monthly bill. I carry a .45 caliber Glock model 37 with me pretty much everywhere I go so I am not too worried about portability of a larger iPhone or for that matter someone trying to steal my iPhone either. They now sell combination holsters for your phone and gun though I have not seen any in person. I love my Itac defense series holster though so unless Itac makes one I am not interested.
You need to identify what you really want:
You want to phone and do basic tasks on the go: get the iPhone.
You want to surf the web comfortably: get the iPad. The 7.9" iPad mini is already on the edge for surfing the web. A 5.5" iPod (or iPhone if you like) will still be a bad device for surfing the web. It will only improve the keyboard experience.
You're mixing up two different devices with different purposes here. "On the Go" versus "In the Couch". Only Apple seems to understand this.