I must confess that I have absolutely no idea what the technical difference between the 600 and the (clearly) lesser regarded V550. My Dad has a V600 and has had great success scanning photos, etc. So I bought that one.
I scan a lot of refractors and prizms which can be quite a challenge. The ACDSee software I got for $29 is really amazing for editing these cards. IT can rotate to 0.1 degrees which sounds ridiculous until your cropping the prizms that always have a little bend in them. If your ever interested just let me know. I've had a lot of fun trying to scan and edit some cards. Some I have just accepted that that's as good as it gets.
Have fun, Greg