Synthetic motor oil is better, period. The more you change any type of fluids, whether it be engine oil, transmission oil, brake fluid, etc. the better it is for your vehicle. Mix in the cost, and that's another story.
My friend owns a Porsche 911 turbo, and he changes the engine oil with synthetic every 1500 miles. Then again, my dad has a Mercedes Benz ML430, and he follows the engine diagnostics, which told him to change at 9,000 miles.
Bottom line - how much do you love your car and how much are you willing to pay?