A few years ago I looked into upgrading my car headlights and researched the subject a lot.
LED headlights are still very rare.
If I remember correctly, last year The Caddilac Escalade Platinum was the first to offer LED headlights.
If you did find any LED aftermarket headlights I would avoid them. Its tricky to design them with enough heat sinking to remove the heat and also to focus the beam properly. If you did buy a LED headlight kit and it failed on the road, where will you get parts?
If you are looking for a "bulb replacement" type of improvement, some of the brighter bulbs (Sylvania Silverstar for example) might offer some improvement. I tried this on a minivan we have and it did help. The down side is that the brighter bulbs usually only last about a year or so.
HID lights are brighter but require a different reflector. They do sell HID bulbs to replace regular types but they don't work as well as complete HID headlight assemblies with the proper reflectors. Also, HID bulbs in a headlight assembly not designed for them was technically illegal as of a few years ago, don't know if it still is. Best bet would be complete HID headlight assembly, but they usually only offer these on a few popular car and trucks models as aftermarket parts. This being said, I know of people that have used HID bulbs in regular housings and are happy with them. Just be sure you are not blinding oncoming drivers. Also, some HID bulbs don't have Lo and Hi beams, only lo beam so make sure you check.
Steve