I don't know the CAP580, but I imagine it wants a side-mounted engine, which would argue in favor of the two-stroke. The YS can be side-mounted, but I think it's better suited to an inverted mount, as you would have with the Venus or other pattern type model. YS63 would be good power for a five-pound up to a six-pound total weight model.

If you've been flying 20 years and are new to four-strokes, the YS is a great way to broaden your experience. The YS is supercharged, which means it gulps in more air than a non-supercharged four-stroke. The more air to burn is the reason for the high nitro requirement. The pumped & regulated fuel system is different from what you're used to with the two-strokes, but I don't think it's significantly more complicated, given that you have the freedom to put your fuel tank on the CG, and don't have to worry about whether it's higher or lower than the carb spraybar. The regulator solves the fuel siphoning problem, and pretty much avoids any flooding of the cylinder. Starts are very easy, but usually do require an electric starter. Also, with the inverted mount, you'll probably want a remote glow hookup for convenience and safety, so you don't have to reach under and behind a spinning prop to remove your glow lighter.