I wondered the same thing, and after a bit of digging I found somewhere (might have been a poster on mnet) that suggested the only difference was that some birthing balls have some kind of safety feature that means if the ball bursts then it goes down a bit slower rather than you landing with a thump on the floor! Tbh I thought that sounded a bit unlikely (especially if you check the recommended height and weight), so I've ordered a normal exercise ball - much cheaper!
birthing balls are often bigger than exercise balls - these are the recommended sizes (found this here
If your height is up to 155cms (5' 2") we recommend a 55cm ball. Between 155 and 170 cms (5' 2" - 5' 8") you should use a 65cm ball and if you are taller than 170 cms the 75 cm ball is the most appropriate.
My exercise ball was wonderful during first stages of labour last time. I bounced on it all day until it was time to go to the hospital. When I got there I found that the balls they had there were too big (I'm only 5'3 and have a 55cm ball) and was GUTTED that I had to stop bouncing! This time I'm planning to have my baby at home and will bounce on my lovely vertically-challenged-people ball!