skaven are a little more forgiving a warband to play, you have access to cheaper troops and a few hero choices. Also their spell casting is done by a non warband leader, so fairly easy and cheap to replace should the need arise. The strength of the skaven is in their numbers, they have one of the highest warband sizes in the game. Making them easy to over whelm an opponent with larger numbers of weaker models. They also excel in speed, just about everyone in the skaven army is high in initiative and in movement.
The down side is their leadership, they start at a 7 leadership and that is their max. So it is fairly easy to force them to run if you can get them to take a rout check. Also the low leadership hurts them in fights against fear causing units.
The carnival is a hard warband to describe, their strength some would say is in their nurgle's rot. That ability can cripple a warband in a campaign. But that doesn't help you in the games your play during the league (what I mean is that it doesn't help you win the fight by inflicting it, it helps in games that follow by weakening the units toughness), so I find it less than helpful. They also have tainted ones, which are like the mutants of the possessed, only they can not take more "gifts/mutations" after creation. This makes them expensive to outfit if you go crazy with abilities. The rest of the warband is fairly straight forward. The brutes hit stuff hard, the deamons are all deamons, and the brethren are then so you have a henchman that can gain exp. Which is the downfall I think of the carnival, not many of thier units get better after games because they don't gain exp. Which makes them great to start but causes them to pale in comparison to henchmen of other bands around your fifth or sixth game.
its really up to you what you "like", because sadly in mordheim no band is perfect. If you excel in something you tend to be weaker in another area. Or you can be average across the board.
