My trike experience is very limited, but I've started shopping. I did a test ride today on a Catrike 559 that had schwalbe crazy bob tires on it. 2.1 wide in the front, 2.35 wide in the back. It seemed like a pretty cushy ride and didn't seem to slow me down all that much. Any experienced folks care to offer an opinion? I'm always going to be one of the slow crowd (12-15mph avg). I'm wondering which would be better for me, a Dumont with skinnier tires, or a 5.5.9 with bigger tires but 3 lb lighter? Or are the standard marathons on a 559 good enough around here?

One of the people I ride with owns a 5.5.9 with standard 40 mm Marathon Racer tires. She rides with them inflated to 80 psi.  They roll fast and provide acceptable ride comfort. She said she rode on some loose gravel once and they performed satisfactorily.
 
A set of 54-406 and 60-559 Crazy Bob tires would weigh 2.4 pounds more than a set of 40-406 and 40-559 Marathon Racer tires. A set of them mounted on a Dumont would bring its total weight to 45.4 pounds, which is 5.9 pounds heavier than a 5.5.9 with standard Marathons. That extra weight, particularly in the tires, would negativly affect acceleration.
 
I have ridden with two other 5.5.9 riders who like their trikes. Don't know any Dumont owners.
 
In light of the above, Patti's comment concerning the Dumont's front suspension, and a $1,000 lower price, IMO the 5.59 with standard Marathons would be the better buy.
 
Some tire rolling resistance factoids: www.schwalbetires.com/tech_info/rolling_resistance
 
Some side notes:  
 
My VTX trike, which is equipped with 23 mm Schwalbe One tirees pressured at 100 psi, is a real bone shaker on the poor pavement sections of the White Rock Creek Trail.  
 
Put a 700 trike together for a friend. It was equipped with Marathons. Took it for a test ride without checking the tire pressure. The ride quality was very good and the trike rolled fast. When I checked the tires, found out they only had 20 psi in them.