It's impossible to answer that, while there are obvious pros and cons, the biggest factor is personal preference and how one's body mechanic produce power.
Someone that relies on timing may not need the longer backswing with more potential energy, and can easily outhits one who relies on brute strength and longer backswing.
Also, it depends on the shot. Service returns, compact swings are better. Volleys should have zero swing. Picking stuff up off the ground should have little to no swing. Blah blah blah.
You can't just have one type of swing for your entire game.
But if you have to ask which is better on any given shot, I'm going to say shorter backswing is better. The game of tennis and sports is to keep things simple: the less moving parts you have, the less likely you'll make a mistake.
Last edited by Say Chi Sin Lo; 09-26-2012 at 08:38 PM.