I haven't owned an Old Town Discovery, but I did have a 14 ft. Old Town Guide canoe that I rigged with a center seat for solo paddling. I don't have a lot of time in fishing out of kayaks yet, but based on what little I have learned so far, I would go with a kayak instead of a solo canoe. Reasons:
1. Kayak is a lot more maneuverable. Now, that's a tradeoff - a canoe will track straighter on long paddling trips on flatwater. But if you're fishing rivers, you want something that you can point exactly where you want, quickly, with a minimum amount of paddle strokes. You will work harder to steer the canoe, and if an unexpected current sends you sideways, it's harder to recover. That can get you in trouble in fast water.
2. Solo canoe will be a little more stable than the kayak, but not enough. My Old Town was pretty squirrely. Your center of gravity is higher in the canoe (provided you're not standing in the kayak, which I think is not for me).
3. Kayak will be easier to transport.