Both are scenic and enjoyable, but the Windermere is flat if that makes any difference. I think a factor, too, to consider is timing and how your training schedule would work for either. The Windermere is a point to point, meaning there is transportation coordination considerations, but it is nice to run to a destination. The CdA is an out and back, so the start and finish are in the same location - better for spectators and coordination.
I don't think you can go wrong with either, it is just personal preference.