If you are into serious fishing/depth finder--you will always benefit from 1000 watts over 600 watts when you get over 500 feet depth--and maybe even at less than that. There will better echo return, and thus better "reading" by the sounder.
I don't know what Ray Marine currently has in relation to the others. The "gold standard" for many years has been Furuno. I suggest that you read the PDF which Furuno puts out about their sounders and transducers.
A few years back hummingbird came out with the side scan/down scan, and Lowrance/Navico has improved on that, with the broad band and side structure scan. These are limited to about 150 feet, especially in salt water, so may not be of interest to you.
Another factor is going to be the transducer--the 1000 watt Ray Marine are going to be pricey. There is a bronze 1000 watt transom mount--and the sticker price is close to $800 for just the transducer. You could also use the shoot thru the hull--if you are willing to do some glass work--remove the core, work in the sides of a "well" with new glass, and then build the sump for the sounder/epoxy or mineral oil as a media to transmit the sonar impulses. The core blocks any sonar wave, so must be removed. A thru hull transducer can be done in a C Dory, but is often not practical because the boats are trailered....Again--it can be done, but is a bit more difficult.
I have never owned more than 600 watts.
