the main diff is most of the cheap kits are 35w and the dearer kits are a 55w, the 55w are a little brighter iv found this when out with a ne rocco last week
iv got a 35w kit and had no problems but i do still have to use high beem on country roads but the 55w i really dont think you will ever need to... (but thats just me)
also if you have to replace the bulbs you have to make sure there 35w ones, most car shops do them for the 55w so better to get them on fleabay as most of them are 35w
another plus, a normal 35w pack is much smaller than a normal 55w pack...