They are basicallly the same game but with updated graphics, weapons and such.
Depending on who you talk to Source is better.
If you buy HL2 off of http://www.steampowered.com
then you will get cs:s right away. But if you get a HL2 retail copy you will get CS:S with that as well.
Basically the only Multiplay That will come with HL2 boxed is CS:S