Definitely Saw You at Sinai.
With Frumster, you communicate directly with the other person from the start. With Saw You at Sinai, you have 1 or 2 shadchanim that are assigned to you, and they match you with people they think match your profile. If you see you are not interested in someone they match you up with, you can email the shadchan directly and have no contact with the other person. In addition, they give you a few references on the other person.