I think Cadster explained the differences nicely. You do really have to read the water and plan in C1 or OC1. I think OC1 especially requires you to look at a rapid differently then a kayak or C1. You have to think about how to run it dry, versus bombing holes and waves because a canoe full of water makes it even harder. OC1 does make you a better boater, but the open boat tends to hold most of us to III/IV. However, there are some C1 and OC1 V paddlers out there and they have my respect.
I have just went to C1 from OC1. To me C1 is harder in a different way, requiring you to paddle differently then an OC1 and more demanding from a roll perspective. I want to a C1 so I could bomb the holes and waves, not worry about filling up and to become a better paddler. Also, I was tired of running and dumping water in big rapids, it is challenging for sure.
I have never kayaked beyond a swimming pool. To me, kayaks always felt very confining, can't see very far a head, hard on the lower back, and no leverage on your strokes in comparison to a C1.
There is a group of 15 -20 III/IV C1/OC1ers from Colorado, not many Vers that I know. I know several OC1ers that are finally converting to C1, including me. The hardest part of C1 is sitting on a low saddle and leg/knee pain, but with some of these bigger river runner kayaks (Burn, Karnali, H3:255, Everest) this problem can be mitigated by using a higher saddle.
There is a big OC1 and C1 get together on the Ark this weekend through next weekend, so you might see a few of us, if your on the Ark.