Well, first off, they are both very different games - the Half-Life series is a linear FPS, whereas Mass Effect is a third-person RPG. For the first Half-Life, although it's a good game, I daresay it hasn't aged that well. The graphics are dated, the gameplay feels rather rough, and there really isn't much story to speak of at all - the only memorable parts of that game are the very beginning, the trip into Xen, the final boss fight, and the Gman.
Half-Life 2, on the other hand, is one of the greatest things ever made by man and you'd be a fool not to play it. The level design is amazing, the story is masterfully told, the gameplay feels very fluid (not to mention extremely fun, particularly when you get the gravity gun), and the lack of any character on your part is completely made up by the fact that every other character in the game is nicely developed and well rounded. It truly is a joy to play.
