The first game is fantastic also, but it has some serious flaws in today's context. The controls are unforgiving and feel very digital. There's not much in the way of 'feel' to it, and there seems to be no inertia or acceleration. One small tap of 'W' can send you shooting off the edge of a cliff in an instant. It makes jumping sections especially annoying. Half-Life 2 of course is way more natural feeling and doesn't suffer in the same way, but there are still a few moments where you'll curse the controls for getting you killed. Then again, it's not really surprising given the amount of stuff you have to do in the game. It's a bit different to "go find the glowing yellow RPG launcher, then go over there and hit that tank exactly three times"
Would you recommend playing number one or stick with number 2?