the f3.5 is a newer lens compared to the f4-5.6. f3.5 is a constant aperture through 10mm to 20mm whereas the older one is not. aperture number increases with the zoom from 10mm to 20mm.
for example, f3.5 at 10mm. when you zoom to 20mm, f3.5 does not change. as for f4-5.6, f4 at 10mm. when...