The difference between HID and LED conversion kits is pretty in-depth. To break down the pro's and con's of each style of bulb upgrade we've created an info-graphic.
LED bulbs draw less power which makes them more efficient and produce 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 end up melting a fog light housing lens if made out of plastic.
LED bulbs also have a longer lifespan than HID which makes 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.