I had the same dilema as you just 6 weeks ago. I went ahead and went with the 04.5 500. Polaris has moved a lot of the 6/700 features down to the 04.5's 500 HO. (i.e. straight pull shifter, hillyard fnt diff, tighter turn radius, fuel gauge, 20 lbs less rotating mass(lighter tires) so really about 70 lbs lighter.) I have ridden both and liked the sound of the 600 but that was about the only diffrence I could tell. Oh and the drive train noise. As for the top end, I didn't buy this rig for racing I bought it to hunt with and work here on the ranch. It will still do over 50 and that is plenty fast for this type of rig.
The choice is entirely up to you this is just my .02 worth.

Hope this helps

Rider