So you will see the "big big brother" of 16F84 and 16F628 in mid range chips. And until you see 16f877A is the bigest brother, after that, you will see high end range is a "huge brother" and the dsPIC is the X huge brobrother...
That is, PICs is a family, and each type of PICs, they support some more functions, from more pins, more data memories, more functions.... And the fact that, you can choose PIC from your applications. You can find "Select PIC guide" on
www.microchip.com for more information about all types of PICs. There latest version of 2004 is now available.
I don't know why but from now, I do not support 17xxxx PIC anylonger. So choosing PICs depends on yourself. But you should choose common PICs to begin, because it's easier for your to develop your projects. And I recommend mid range pics.
16F84A used to be the most common chips for beginners, and now it's 16F628A. 628A is cheaper than 628 and 84, 84A, but more functions and I/O pins. They usually recommend to use xxxxA because it's cheaper and newer.
Why they choose 12Fxxx is that they need a small chip for small simple function. Therefore, you shouldn't compare the differences between other types of PIC. 12F has 12bit opcode, and 16F has 14 bit opcode.
If you need 12F for your applications, I recommend u to use 12F675. And if you need smaller, choose the SO package.