Well if u got Source, then you would be buying Half Life 2 aswell, so it seems to be the better value; two superb games for one price.
Battlefield 2 gives you a great feeling of all out war on a massive battlefield. The only problem i find with BF2 is that a lot of the experiance raved about in reviews is when ur part of a squad thats working well togeather on the battlefield. However, people in online games very rarely organise into a few sets of full squads, and hardly ever work with or take orders from the squad leader. So most of the time in online games you can feel kinda lonely, trapsing across expanses of the map to get to the next enemy base on ur own, only to get nailed by a group of enemies spawning there. In the online games the thing that is supposed to make BF2 so great, the feeling of a squad working and fighting together, is very rarely there. It sometimes becomes a bit tideous wondering round on ur own, or trying to work with dumb squad members who have no intention of working as a team.
Through my experiances with CSS, the teamwork feeling seems to be more alive, it may be because the maps are smaller and every CT or T spawns together, so you're more inclined to stick together. It is also because CSS is realistic in that you cant run into a base all guns blazing like you can sometimes on BF2, because a few shots and you're dead. Teamwork is key on CSS, and although the maps are small, with no vechiles or stationary guns, the sence of teamwork and the fierceness of the fights seems to outstrip those encountered in BF2