Fatter tires will help, but they're not a complete substitute, either. You'd gain a couple millimeters of travel & shock absorption but lose a bit of speed (not really important in town, though); a suspension fork will always take bigger hits. You'll also lose grip & traction in most conditions (everything besides ice & snow) if you choose a tire with knobby tread.

I don't have to deal with very many potholes where I live (having said that, I'll probably nail one today.. lol) -- instead, it's mostly sloppy patch jobs, cracks, and occasional heavy steel plates. There are definitely times that make me consider selling my fully rigid Bad Boy Disc and upgrading to an Ultra. I already have another bike with an entry-level suspension fork, and although it's nice when the surface is rough, it's just not as fast & responsive as a rigid bike, I can't lock it to behave like a rigid fork, and its suspension action doesn't work as nicely as the Headshok's.

I know that you said you're keeping your Trek, but you could sell it and make up most of the price difference between the Jamis and the Cannondale.

Another option would be to get a suspension seatpost. It'll take most of the annoying buzz & bumps and save your butt. You'll still have to "ride light" over really rough pavement (lifting over potholes, keeping your grip on the bars light, etc). But, the ride will be smoother, and you'll still have the efficiency of the fully rigid bike. I also feel that a suspension seatpost can help save the bike from the shock of your weight -- that is, it can hit a bump and move just a bit before having to move your body weight as well.