Light vs Heavy units are all about the types of vehicles they use. So a heavy unit will have armored vehicles, such as a Bradley:
Vs a light unit would utilize light/wheeled vehicles such as the Humvee.. Light units also tend to do more dismounted operations, given that the heavy vehicle is such a force multiplier, you want to keep those vehicles up.
Most Cavalry units have been moving over to Strykers, which are wheeled vehicles, but heavier and more armored than Humvees, but lighter and less armored than a Bradley.
In terms of the break-downs, I believe most of the Calvary Regiments have moved to be predominately Stryker.
Mechanized (Heavy) Units that I know of include 1st Cav (avoid this unit if possible), 1st Infantry Division, 3rd Infantry Division (although they do have Light Infantry as well)... 4th Infantry Division has elements of Stryker, Light, and Heavy... 2nd ID is all Stryker...
3rd Cav and 2nd Cav are both Strykers.
Light Infantry will be units like 10th Mountain (excellent history with that unit), 101st (Air Assault, if you can get this unit, I'd do it, but not sure if you need Air Assault in your contract, my guess is no).. 82nd (Airborne) 173rd (Airborne).. Obviously units like the 75th Ranger Regiment would be specialized light infantry.
In terms of the difference, it's going to obviously be the types of vehicles they use (heavier more complicated vehicles means more work for you.. I'd see heavy units sit in the motorpool fixing vehicles every Monday, while we would check out our Humvees and leave, very easy to maintain. Additionally, light units will do more dismounted operations, so there's more physical challenges to those units, having to hump everything on your back in and out.. Personally, I think that's more fun though. I was someone who didn't like to be in a vehicle all the time, I wanted to be on foot.
Both will do some dismounted operations obviously, both will use Humvees (even heavy units, they often rotate depending on the mission).. Units like 10th Mountain, 101st etc, (lineage units) tend to get more deployments, and light units are easier to mobilize and deploy because there are less logistics to get in the way.
I'm not super familiar with Cav things, given that Rangers tend to be more excluded, but we did do missions with other units at times (we'd have them in blocking formations while we assaulted) and would talk to them every so often, so you can get a feel for it. Pretty much everyone I've spoken to say that 1st Cav is the worst, so I'd avoid them. They don't do missions, they don't hold territory, they just kind of suck..
If you have specific questions on anything, feel free to hit me up, I've done a lot of schools, including RASP, Ranger School, Airborne, Air Assault, Sniper, and Military Free Fall, I'm happy to help as much as I can.
