35w is the standard OEM ballast wattage used by all car manufacturers. A 55w ballast will make whatever HID bulb you choose slightly brighter but will reduce the overall life span of the bulb. I personally have a 35w ballast and it looks fine.
Drewkow is right. 55w will give you more light output typically, 20-25% more light. The downside is you will have a decreased lifespan of the bulb and ballast, which will last typically only 3-5 years versus 5-7 years @ 35 watt. But if you are a mechanically inclined person, 3-5 years is still a long time to pass by before working anything on your car.