Sleighty, I have both games and as you know I also play a little BF

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HL2 Retail version - Doesn't come with mp yet but is rumoured to be available through Steam as bonus next week. It does have Counter Strike: Source which is mp.
HL2 Steam Silver version - This is the one I purschased as it has the above + Valve's back catalog but will also include when its available Day of Defeat: Source (WWII).
I am looking forward to Dod:S as it will more then likely be my next main FPS. The HL2 engine has a great feel imo. Dod:S is due for release end of 04 or early 05.
Call of Duty - Its not to bad but I'm not a big fan of the feel of Quake 3 modded games but its still a fun game, it just didn't grab me like BF & HL2 has.
Imo its better to buy HL2 through Steam online as you get more for your money.
HL2 Retail version = HL2 and Counter Strike: Source for $70
HL2 Silver steam version = HL2, Counter Strike: Source, Day of Defeat: Source + Valve's back catalog (total of about 14 games) for $85