Wondering which one I should get, since each game is $10
Source is not much of an improvement over Half Life. I would personally get Half Life for nostalgic reasons.
II haven't actually tried half life source, but black mesa is out which is the first half of the game.
Go for the original Half-Life. Not only does it have the nostalgia factor like Spiral said, but there are also plenty of great mods for it, far more than there are for HL Source. And I'd highly suggest you also try out Black Mesa afterwards. It's a fan-made remake of Half-Life made entirely from the ground up in the Source engine. It includes the entire game, except Xen (so probably closer to 80% of the game), which still gives you about 8-10 hours of gameplay.
Buy Half-Life so you can play GoldSrc Mods
Download Black Mesa and play it for free. It's Half-Life remade in the Source engine.