let me put it in prospective:

CS:

pros:
lots of players
low system requirments
lots of diffrent servers
simple yet effective graphics

cons:
a few cheaters
"simple" is another way of saying bland

CS:CZ:

pros:

Improved graphics
low system requiments
new levels, plus old ones remade
Get CS as well
Bots for practice

cons:

smaller community
grapics are better but still not great
Bots play like a 8 year old on drugs or a hacker

CS:S:

Pros:

diffrent graphic engine, much easier on the eyes
remade levels
resonable community
buy HL2 silver package get CS:S, HL2 plus lost coast/DM, DOD:S
Bots

Cons:

Silver pack is $80+ USD
Still some cheaters
Bots could not shoot the sky
some dislike the feel of the new engine
Do you have a super computer?

I personally play CS:S but many say CS is better. If you have a beefy PC and are one for graphics, get CS:S. if you like the idea of the old school feel go for CS.