Can they shoot down cruise missiles, or just planes?
Mainly for long range acquisition and tracking of airplanes, under some circumstances probably stealth too.
CMs would be low altitude and wouldn’t pop up on the radar until in horizon range which is <10 miles... so probably not an S-300 would get one. But there are others like an SRAD SA-15 that might.
The point of the S-300 is that with two, three maybe 4 systems, they could blanket the parts of Syria we’d likely attack (each covering a 300 km circle). It is a plane killer and a very good one. We would not get through unscathed.
Both...the Russian system is one of the best in the world.