Hmph, gameplay is more important then the technical side of games, and what do you mean with "All San Andreas does is let you see what the streets of LA are like without actually going there." San Andreas does no such thing, the entire state of San Andreas is FICTIONAL with some places looking alike, but it's mainly fictional.
Let's not forget the tons of missions, side-mission, and the actual diversity in the gameplay, all you do in HL2 is shoot Aliens, bad guys whatever, in SA you race, dance, break into houses, perform drive-bys, hijack a plane, etc. etc.
San Andreas for the Win.