I would personally avoid the Diamondback. Like other big-box sports stores that don't do anything with bikes but sell them, you're not likely to get great service (in comparison to a quality bike shop anyway) from a place such as Sport Check should a problem that needs warranty work or specialized advice occurs.
I took a good hard look at Norco's VFR Three's before ultimately purchasing my Trek Valencia (a bit pricier at $850 CDN), and can't comment on Kona as a brand from personal experience, but I have seen some pretty sharp looking Kona's and have met a couple roadies who swear by them.
I'd test ride (if possible) your two latter choices and go from there. At a $500 pricepoint those are two quite decent bikes.
Last edited by AaronJohnTurner; 08-17-10 at 07:51 PM.