Both , but i you have to choose : half life 2 . fear is good in singleplayer , half life in singleplayer and multiplayer , and with half life you get GREAT mod's
Having played both, I'd say HL2. Steam and the installation of HL2 are obnoxious as Chemix2 said, and I probably won't be buying any Steam-enabled software ever again. But ultimately FEAR's lack of decent multiplayer kills its replay value. You'll play through FEAR once, maybe a second time on a higher difficulty, since the AI actually gets better rather then just you taking more damage. But then that's it, as there isn't much of a multiplayer. HL2 you'll get one playthrough (all hard does is make you take more damage) but then you have CSS to fall back on. Definitely go with HL2, then maybe pick up FEAR once the price falls.
It seems that you need to get both, cause both of them have a good storyline plus both going to have lots of mods, so you can try both of them once you have the money to purchases these.