Undivided: Although no longer an 'official' Mark, units and characters that are not sporting one of the Dark God's personal brands are classified here. Of course, a number of WoC unit's do not have the option of taking Marks and are also lumped in here. You will rarely see Chaos Undivided Warriors on the tabletop anymore. Sorcerer's are the most likely to be unmarked, in order to give them access to the BRB lores. The other unit that is most commonly unmarked is Chariots, since it saves points and it usually isn't that useful to give them a Mark.
Pros: Access to BRB Lores, Cheap Cons: None, other than not having a Mark!
Common Units: Sorcerers, Chariots, Hellcannons, Trolls
Pros: +1 Attack, ItP
Cons: Most Expensive Mark, Lose Combat and you lose Frenzy, Forced to pursue/overrun. No Magic!
Common Units: Marauder Horde w GWs; Warriors w AHW or Hal/Shield, Chaos Knights, Lords and Exalted characters.
Pros: +1 Cast/Ward Save, Fairly cheap mark. Cons: Tzeentch Lore is mediocre, Opponents hate you!
Common Units: Sorcerers, BSBs, Warriors w HW/Shield, Knights w Blasted Standard, Chosen, Warshrines.
Pros: Good against Shooty Armies (anyone still play these?) and high Weapon Skill armies.
Cons: Expensive Mark, Nurgle Lore is mediocre, Painting Filth, Pus, Buboes, etc.
Common Units: Nurgle Knights with Banner of Rage, Festus in a Khorne Marauder Horde
Pros: Cheapest Mark, Immune to Panic, Fear and Terror
Cons: No longer much benefit, Lore doesn't work against ItP units/armies.
Common Units: Marauder Horse
So there you have it, the take on current Marks. I barely touched the Magic Lores, since I intend to hit each Lore in a detailed breakdown later.