The basic theory of both is that the efficiency depends on the temperature drop across them. The hotter the hot soide and the colder the cold side, the better. However a stirling engine is capable of working, albeit poorly, with lower temperatures, even lower than 100C. Both need up around 500C for really good efficiency. This tends to be steam turbine or multiple expansion piston engine territory, so you are talking of lower tempratures. Generally stirling engines use hydrogen or helium as the internal working fluid because it is slippery, and very conductive so the RPM can be higher, more like the 1500 - 3600 RPM a readily available generator will need. That is also about efficiency, eliminating gears.
Going by commercial units, the overall efficiency is about 25%, so to get 500W you need about 2.5 square meters of sunlight focussed on the heat absorber. As your efficiency is likely lower, probably area makes up for that. Of cource the collector mirror arrangement has to track the sun.
I think the stirling engine would assuming you have good machining resources. Not much to copy off, so it is a big learning curve. A steam engine (piston) with only one size piston will not extract all the energy from the steam, and a turbine needs the same thing, expansion through high pressure into lower pressure parts and eventually to the condenser. Steam probably more dangerous than circulated liquid salts perhaps in the stirling type. The first link below has plenty of reference so you can see where to go. The second link has a video on a solar steam generator system down the page. You may be surprised at the size of the collector for a small engine. You are looking at something like 1hp or 750W engine power. The 2 cylinder plans they mention are up to 10hp, so that method suits.