Well AC I gotta tell you that I have spent a lot of time on the CRF450 and have owned my fair share of 250MXers.. The CRF is an AWESOME bike with AWESOME handling BUT (and dont get me wrong on this, I LOVE the bike!!) - for dune riding there is NOTHING on this planet like a KX500!! I have beenriding a one now for a few years and it has proven VERY reliable, FAST, a GREAT woods bike when flywheel weighted, FAST, has ALL the motor you can imagine for climbing ANY of those power robbing hills at silver lake, FAST, lift the front end up anytime anywhere - right from an idle in 5th gear and BOOM up she goes, FAST, and its CHEAP to run - I got over 8k miles out of a piston and it still looked good - did a topend for under 150, plus did I tell you it is FAST!!!!!!!!!
Not sure about the YZF but I have been told by more then one person that the 4 speed just doesnt work for trail usage.. On the other hand, I wonder if a Revloc would help with this...
Anyway, your best bet as always is find someone who has each of the bikes and try them before you buy them..
I certainly agree with you about the quads, their fun but bikes get in your blood!!!
Woodsy