if the big places and smaller places you are thinking of are both pretty much the same niche, well, the bigger places have lots more traffic. otherwise you're comparing apples to toilet seats because advertising only to people who would be interested would be the thing i was most concerned with, and two other things - i would advertise only when my site had the overall signup ratio i wanted and i would make sure that my banner was kickass with a tested clickthrough ratio before i paid to place it anywhere.