It's too hard to gauge HL2 since it hasn't been out long enough to really grasp the influence it has had on game developers. It's hard to compare the two since HL2 had the momentum behind HL1 to push it forward.
Half life changed the way you play a game because they put such backstory behind it, the interaction with NPCs in that game was absolutely groundbreaking at the time and the fact that the story was a linear real time scenario actually emerged the player in the game itself. While I can respect Valve for trying to diversify the environment of half life, the lame jumping puzzles on Xen at the end really detracted from the earlier parts.
HL2 introduced many unseen aspects into the game, but perhaps what it lacked was the difficulty and time put into half life. The soldier AI in HL2 is laughable and there was nothing to replace those crazy black OPs from the first game. HL2 was moreso storytelling and less gameplay vs. the first. I played Half Life for weeks before I finished the game...HL2 took me about 3 days
Now, if HL2 had been released a year earlier when it was expected to, it would no doubt have been revolutionary for that time period and I may have a different opinion. Don't mess with success though..HL1 > HL2 and if anything, Black Mesa: Source will convince anyone who favours the technological edge of HL2