The Chaos codex is very deceptive as it has only two entries but with selecting a character with a specific mark you can shift units around in your book including all four cult troop choices and Chosen (if you take Abbaddon).
Chaos Space Marines: Some people have tried to compare them to a Grey Hunter, which is like holding an apple to an orange. You get something different from each in terms of tastes and nutrition, that can be said in this case. For 2 more points they get a close combat weapon matching the cost of a Grey Hunter, but then this is where it gets different, they can take two special or one special and 1 heavy weapon, their Aspiring Champion makes the unit have a decent leadership (add for another point Veterans of the Long War) and perhaps a mark for a few points more and the unit can be built to suit your needs. A CSM unit is a nice filler between a Cult Troop Choice and a Cultist Troop choice, so keep that in mind.
Chaos Cultists: Cheap, massed troops and with the right leadership hard to break. These guys are great to hold objectives, swamp targets with massed attacks and Marks are cheap. It should be obvious how to take cultists and use them on the table. The favorite of late seems to be taking Typhus and upgrade your units to Plague Zombies, now you have cheap scoring, horde units that are fearless. Yes please!