I have never owned a Scag, but I do own a IS100Z. When I looked at purchasing a ZTR, the Ferris seemed to have more features, and the Kawasaki engine that I wanted. It has been a great machine. I don't think you can get the Kawasaki on the 2004 models though.
Get the Ferris. Ferris mowers are the best in the industry and they are built like a tank. Also my experience with Honda engines is not that good. I only used a 13 hp on a 48" mower but the darn thing kept breaking down. Kawasaki engines have a good reputation.
Another thing to consider, and I don't want to start a flame or offend anybody, is dealer support. The dealer where I bought my Ferris has a policy that if the machine can not be fixed right away they will give you a loaner model to keep you up and running until your machine is repaired.
I don't know if other manufacterers provide this support, but this was one of the things that led me to the Ferris.