My theory on the shift to the high take back: Slower courts.
If you played on fast courts, especially grass, you'd want a pendulum takeback to deal with low and skidding balls and bad bounces. In addition, when the opponent is approaching the net and volleying a lot, a quick 1hbh takeback is ideal for passing.
all modern players (Fed, Wawrinka, Haas) started with the racquet standing up (head level), then drop down to the side of the body, before swinging forward.
James Blake has a pendulum take back. (So did Sampras, btw).
Also, look up Dominic Thiem. He's was a talented junior who recently turned pro (he's 18 now, and is 0-1 on the ATP). He uses a pendulum swing along with a continental-ish grip, held in the hammer position (heel pad further back, fingers bunched together). Hopefully, he'll make a dent in the top 50 in a few years.
Whereas, Lendl, Boris, and Edberg all have the pendulum takeback, meaning the racquet was never at the height of the head level, before swinging forward.
The height of the takeback doesn't define a pendulum swing. In fact, Edberg brought his rackethead up very high on the backswing, often above his head.
Check out his first bh in this vid:
http://www.youtube.com/watch?v=qoDFcHnWZR0
I think the key difference between the two strokes is the position of the rackethead during the unit turn.
So which takeback do you recommend? My problem is that I have a hard time keeping the racquet facing sideway during takeback. My tendency is to lead the racquet facing the back fence even before the racquet drop. Am I turning the shoulder too much?
I'm not sure what you're describing, but if you use the pendulum, take special care to drop the rackethead into the power position (racket horizontal) before the forward swing. One virtue of a high loop is that it more naturally drops the racket into power position.
In deciding between the two, another thing to take into account is how you like to hold the racket in ready position. If you use a pendulum, you should be comfortable with a low ready position.
I just don't get the same rhythm by taking the racket head back low and then waiting for the ball.
Ideally, there shouldn't be a waiting. You would time the backswing so that it naturally falls into the forward swing with the same rhythm of a pendulum. Of course, you won't always be able to achieve this.