There are a few things to consider when trying to determine if a 35 watt or 55 watt HID system is better. The most important factor is probably the amount of light output, or lumens, that each system produces. In general, a 55 watt HID system will produce about 3200 lumens while a 35 watt HID system will produce around 2600 lumens. So, all else being equal, the 55 watt system will be brighter.
Another consideration is power consumption. A 55 watt HID system will obviously consume more power than a 35 watt system. This is something that needs to be taken into account when deciding which size HID kit to buy, especially if you're worried about running into problems with your electrical system.
Finally, it's worth mentioning that different manufacturers produce different quality HIDs, so it's possible that one brand's 35 watt kit might outperform another brand's 55 watt kit in terms of both light output and power consumption. It's always best to do your research before making a purchase.