Obviously I'm an Idaho boater, and maybe I'm a wuss but I wouldn't trade my dry suit for anything. I use it late October - May. I don't screw around with the possibility of hypothermia, so I try to minimize the risk as much as possible. It's been essential for any winter and early season boating, and I loved having it on November Westwater last year. Never done the Grand so no advice there.
That said, I don't catboat (I'd like to!) so I can't tell you the differences; and plenty of my friends get by with the drytop/layer combo even in the winter.