Why are you looking to upgrade? The Prizm II will serve you just fine and you will find lots 'o goodies with it. You might want to keep it for 6 months - beat the parks to death, collect your clad and silver and use that and the sale of your II on ebay to finance a more expensive machine if that's what you're heading for. You may find that the Prizm II suits your style after all.
I will chime in on the Prizm V. I've had it since x-mas, and been out only a few times since it's butt arse cold here in Michigan. What I can say with my minimal experience is that it is very easy to use, light to swing and pinpoints like a charm. I've already struck silver, along with several dollars in clad in just a few times out.
Now, for the extra hundy you get a few more bells and whistles, plus the bigger coil than the IV. Some say that the Prizm IV is actually a better deal - that the extra tones aren't necessary and the bigger coil doesn't get you much extra depth. I guess its just a personal preference - I like the idea of the extra tones and a little more depth. Plus the VCO pinpointing is worth a little extra in my opinion.
Biggest problem is that the prices are generally fixed by the manufacturer, so expect a $399 for the Prizm IV and a $499 tag for the Prizm V. I actually got mine for $450 from a local dealer - who was probably not suppose to sell at that price but did anyway. You can usually find used ones on ebay for around $400.
Good luck!