You can pretty much get the same apps on the iPod as you can the iPhone. If you get the iPhone, you have to buy a plan through AT&T, or whatever company happens to carry it. But anyways, you need a plan. With an iPod Touch, you just need Wi-Fi to access the Internet, but it's not required for anything that doesn't need the Internet. It's not as fast as using a laptop or desktop, but that's to be expected. There are also some things you can't do with it, such as view Flash on the Internet. But, for general use, it's probably worth it. I've even used the CAA chat with mine. I don't think I've ever tried Kotoba before, but the touchscreen is usually pretty good. My 2nd gen, which is a couple years old, still works great (I usually keep it in a good case and put a screen protector over it, and overall don't abuse the thing; I recommend doing that.).
Anyways, I suggest you go for the iPod over the iPhone.