What has the best 'societal' benefit, fostering or adopting?

I'm less than a week into bringing my first foster kitty home and I'm already thinking about adopting her. One of the reasons I really wanted to foster is to help get animals get off the street/out of the pound, warm and safe in my home, and spayed/neutered.

If I adopt her, I won't be able to help more animals. And there will be one less foster home available (therefore, one more cat in the pound).

What's the best thing to do, emotions aside, to help animals in need? I realize it's likely location-specific, but is there a greater need for fosterers or adopters, generally?