They all convey the same information from one device to another, but s-vid. is the lowest impedance connector with generally better shielding, depending on the quality of cables, with RCA connections providing the highest level of resistance. Don't concern yourself too much with it if you have a less than 5 foot HD screen or less than 4 foot run of cable.
I have a 6500 lumen Eiki projector with a top of the range Extron director; I use the RGB connection on either end and transmit through Belden Cat 5 No Skew with (2) +10db amps because I can fine tune the phase timing most easily with this setup, but most people aren't viewing a 15 foot image.
Non Tribuo Anus Rodentum and off to the next adventure (RIP)
So, as I'm understanding this; My 27" Sharp with a CRT and a < 6' cable run will do just fine with an RCA video line and the expense (however marginal) of buying an S-video cable would yeild equally marginal improvement in picture quality.
The odd thing is, both components, TV & DVD player use RCA style connectors for the RGB, not the cool bayonet connectors I've seen on some RGB cables.