I played them in succession and HL2:Ep1 is vastly superior. SiN:E is fairly mundane and lacks the charm and charisma of Valve's title.
They are both fairly short, but Ep1 is good fun all the way through pretty much, whereas SiN:E is no better than plenty of older shooters, which cost the same or less and are much longer. I mean the likes of Doom3, Far Cry, NOLF2 etc can all be picked up for under £10 these days.