Seriously, they're all great bikes so it's a matter of nuance as to which one you choose. My proposal is that you take each option for a test ride if you can, then decide which one you like the look of most.
What do I want to do... mainly ride for fitness but also look at the odd race like the R4 or Rotorua Flyer etc. Fairly new to the game. As bikes aren't cheap I figure I'm best to get a pretty good bike in the first place so I'm not constantly upgrading gear or having to change bikes in the not so distant future. From my other sport I've found it's cheaper in the long run to get the good stuff at the start. I like the specialized stuff too... but a fair bit more expensive. I have set myself a limit !
So far I've ridden the Krypton and the Cadent so far. The Cadent being alot harsher ride. I expect the Quantum to be harsher again... which I'd accept if it was noticably faster/more responsive. So currently I'm tending towards the Argon. I believe the carbon bars absorb alot !!!
Looks wise I like the Avanti's curved top tube and tube shapes (looks like alot of specialized influence) but quite like the Argon 18 brand in being a bit more unique. So I guess it comes down to which is the best bike...