premier vs regular pioneer are exactly the same
you will appreciate the clarion if you get a chance to compare it to the pioneer. it simply plays the music better!
the clarion vs. the alpine comes down to ease of use. the clarion is much easier to tune properly.
also, the clarion is cool because it is somewhat unique. its name which carries a certain degree of prestige- which is always nice, but also its not something you'll see in everyone elses car. which also adds to its appeal imo.
also, it comes with auxiliary inputs, and you can turn the internal amp on and off (nice if you have amps for everythign- and just another example of its flexibility and attention to design).
I've owned 9 different hu's in the last 6 years most from pioneer. I've also installed many, many others. I had never tried a clarion until i decided on my own 945dxz this spring partly due to another forum members testamonial that it would have a more full/simply beautiful sound, and i completely agree. I have since installed 4 clarion units in other peoples vehicles. I will never go back.