I agree that HL2 Ep 1 felt a bit short -- brilliant, but short. It's hard to make a direct comparison between the two in length, but I have to say that SiN: Emergence is probably one of the best $20 I've ever spent on a video game, because the replay value is huge. And now that they've added Arena mode, it's even huger. And when Multiplayer comes out... well, it'll be hugerer still!
Uh, yeah. Seriously though. I played HL2 Ep1 twice through and enjoyed it immensely both times. SiN: Emergence, I go for a spin on Arena Mode almost every day and I've played the game through more times than I can count, including several attempts at Hardcorps mode. HL2:Ep1 is very dramatic; whereas SiN is just plain *fun*.
Does that help?