NO NO... you misread the OP, he is going to play BOTH games, he wants to know which one to play FIRST.
I picked Doom 3, cause its CRAP. Its better to play HL2 second, it is leagues ahead of Doom 3 in gameplay, immersion, scenery... etc, so get Doom 3 out of the way, then spend the rest of your life playing HL2 and its myriads of mods
What you could do is install HL2 first then open it up and let steam start decrypting it, then install Doom3. You'll probably be finished Doom 3 around the same time that steam finishes updating and decrypting HL2, lol, so just in time for you to dump Doom 3 off your HDD and free up all that space for CS:S maps and HL2 mods w00t!!!