heres a break down I wrote a while back for agents.
http://forum54.oli.us/index.php?topic=326.msg6536#msg6536cartel has a few but great grunt units.
In my book peace keepers are one of the best grunts. 20 AR, well armed and at a nice Point cost. They do one thing and they do it well..Shot and then Shoot again. Dam 10 rifle, with long range, combined with a good RC 10 just means alot of well place shots that hurts. Even their SA Fast shot is all about getting the shot off before the other guy does. find them to be a great basic squad. I've fielded whole army of them with great success.
a squad of 6 to 8 is a strong unit.
Mercs. I find are only good for their flame thower. Never had much luck with them. Others may feel different.
Cartel Police. now this is a neat unit. Infilitrate and a smg, very kool combo. These guys are my sledge hammer. very good short range killers. damn good armor (spent the point in armor up grade) with their SA they can and will get in close to where their smg's shine and the high AR means they dont go down quick. Their also grunts you can have whole infilitrateing army...maddness (this something i must try one day). I've grown to really like the cops.
Snipers shine at med range or shorter...though they can their not at their best at long or extreme range. Dont get caught in a sniper deal, but neat unit to have none the less. I take 1 to 3 in most cartel list I play.
for models I use are vor union grunts for peace keepers and Union power armor for Secruity force(cops).
On a side note I havnt used doomtroopers yet so let others talk on those.