Bethpage will do wonders for your short game if you're just starting out because you'll barely ever find the greens.
However Oakmont's heavy bunkering and sloping greens will probably give you the best overall game as it tests all aspects of your game and if you're not on the button you'll be punished. Playing Oakmont can be demoralising at times but stick with it because it will help you out later. And when you do score a good round there, it feels that much better knowing that it's probably the hardest course on WGT.
Jack.