First I'd like to get my definitions of the swings down just so we can get on the same wavelength and for anybody else reading this post can understand.

Natural Golf Method - This is actually usually referred to the method Moe Norman used with the hands on the same axis as the club. However, I believe it is NOT the natural golf method you are referring to.

Old Method - This is the method I believe you are referring to as is more of a weight shift back, turn, weight shift forward and turn.

Modern Method - This is a "rotary" type swing....a turn back and turn through swing.

I'll ignore the Natural Golf/Moe Norman method since I don't believe we are talking about that here.

The pros of the old method I believe is it is indeed more natural. If you have ever played baseball, you do need to rotate the body, but you also push off the back foot when you swing the bat. Also the old method is probably a bit easier for those with weak flexibility.

The modern method I believe is far better provided you are doing it correctly. However, that's unless you're doing the stack and tilt method with is IMO...a bunch of crap. The stack & tilt makes the swing shorter which is one of the reasons why I don't like the technique. However, the modern rotary swing has almost nothing to do with making the swing shorter. Swing length is mostly determined by the amount of shoulder rotation. So you can use the old method or the modern method and if you have the same amount of shoulder rotation, you're likely to have the same swing length (unless you are breaking your wrists or taking the club too far inside with one swing). A lot of the modern swing is based off of how Sam Snead's swing and his swing was quite long (http://www.youtube.com/watch?v=U6Gu7n7Vnm0). Of course his swing was long because he had an amazing 140 degrees of shoulder turn.

The great thing about the modern/rotary swing is that not only can you hit the ball further, but you can be more accurate and consistent. You're using your big muscles in your body like the torso and hips instead of the small muscles like your hands to hit the ball and thus you are also more or less returning the club to the ball correctly instead of relying on your hand and eye coordination to hit the ball correctly.

IMO, hand/eye coordination is important but VASTLY overrated in the game of golf. Very few tour pros have major flaws in their swing. Instead, they are usually VERY technically sound and they cut down them having to rely on hand/eye coordination in the swing. And the ones that do have a lot of flaws make up for it one way or another. Vijay Singh is a great example. He has a lot of major flaws in his swing, but he also has incredible hand eye coordination. If you have ever seen Vijay swing LEFT handed with a right handed club...it's pretty incredible. He also practices like a madman. The rotary swing, if reasonably executed, will allow a golfer to not rely on great hand/eye coordination and massive amounts of practice time.

If you're going to the modern method, I would suggest getting into a full stretching program...something that stretches things like the neck, arms, shoulders, upper back, lower back, hips, hamstrings, and calfs.

As far as swing length goes, it should be the same with the modern or old method unless you're using the stack and tilt or unless you were previously breaking the wrists more with one method than with the other method.