The difference between HID and LED conversion kits is pretty in-depth. To break down the pros and cons of each style of bulb upgrade, we've created an info-graphic.
LED bulbs draw less power, making them more efficient and producing the same light output brightness as their HID counterparts. This makes them particularly useful as a replacement for your original fog light bulbs. Many fog light housings are not very big, and if you increase the heat in that little space, they can melt a foglight housing lens is made out of plastic.
LED bulbs also have a longer lifespan than HID, making them a better buy in most cases. One way that HID is better has to do with color and power level options. Generally, LED bulbs have 1 color and 1 power option. The HID kits have 2 power options and 5-7 different color options.