6 months ago i bought a cannondale r600 - i tested the trek 1200, trek 1500, specialized allez sport, giant ocr1, 2, 3
hands down the cannondale was the most comfertable, and speedy of them all.
i found the trek 1200/1500 were too twitchy, and the specialized allez just felt wrong for me.
Its definatly a personal issue. Ride them all, and take what feels best. the 1500 is more expencive than the r600 was and although i had no problem paying a extra 200 bucks for the upgrade, i found the cannondale r600 was just better for me.
The r600 is a 2004, were as the 500 i beleive is essentially the same, only the 2005 or 2006 model. let it be known, that my r600 came with ultegra rear derailer, and front derailer, as well as 105 brakes/front shifters - i beleive the 500 is tiagra
-hope this helps
Cam