I think a lot depends on how much you want to spend. If you have a truck, there's not much difference in loading it yourself between a 10 ft. boat and a 12 ft. boat. You'll appreciate the additional space, even if you're just puttering around. They paddle better in a straight line, they hold up better in the wind, and they keep you higher up with less chance of dragging. At 5'10" I think you'd really be fine in a 10' boat, it's just that a 12' boat is a little better at everything except price when compared to it's 10' counterpart.
Assuming you're in Louisiana, I think you're right on point in wanting a sit on top. They're just easier to get in and out, and being higher up isn't usually an issue.
I think that pescador above is a pretty solid deal if you're not sure how much you're going to use it. The sun wear is clearly visible, but if the hull is fine (and assuming the dude's been paddling around in sandy bayous), it probably is, that's a good deal. I had that same boat and loved it. Only complaint was the seat. There's not much cushion there, but that's a small complaint, especially if you're not going to be doing any overnight trips with it.