It's a question I have considered for years. I always thought that face-to-face was an opportunity to get personal spiritual direction from a priest who knows that it's me in the room with him. Today, though, I confessed at a church with no option for face-to-face confession, and I gotta admit, the screen picked up a few votes today.
First, Father was able to give me extraordinary spiritual direction that was far more relevant than I expected. It was right on, actually-exactly what I needed for my spiritual growth at this point in my life. At first I was totally surprised at this; but as I thought about it, it dawned on me that sin is sin: one act isn't any different committed by me than by someone else. A priest doesn't have to know who I am to know that sin is damaging my soul.
Second, I didn't feel the need to "clean it up" for Father. The screen completely took away any desire to hang onto vanity or pride by withholding things from a man who might think less of me after I've confessed. Granted, I've been told by several priests that they instantly forget what they hear in the confessional; but that doesn't exactly make the process of telling a holy man to his face that I've done something sinful.
Fr. Ed (a priest I know fairly well) has said that we've done our children a great disservice with face-to-face confession. When children are small, their sins are small; but as they get older, it's harder for them to admit the harder sins. Encouraging a face-to-face confession as the better option can further burden us and keep us from the sacrament. Remember, deliberately withholding sins while confessing is far worse for our souls.
One more thing: if you were taught that your face-to-face confession is "more valid" or "better" than from behind the screen, forget all about that. There is absolutely no difference in the end result. What matters is that you've admitted your sin, and resolve to not do it again, and you don't have to do it sitting in a chair staring at the priest.