I'd say go with minecraft, cs source is a great game but its a typical shooter. minecraft is actually something unique and will keep u entertained for hours on end if u enjoy building and creating things
There are plenty of free shooters online. Don't even bother to get Counter Strike! You can get AVA (A fun F2P MMOFPS)! Better graphics, better gameplay (Since it is like CS and CoD combined!), and its FOR FREE!
Have you seen any MMO building games lately for free? NO (exept for Roblox, but it is not the same.)!
So overall, I recommend you getting MINECRAFT!!! You can also go to minecraftforums.net to download lots of cool mods to have your singleplayer or multiplayer experience much FUNNER!