Space Marines vs. Chaos Space Marines

[Kaptain Badrukk]

Thank you all for your advice! I have searched through the Internet and various wikis, and the chapters that have caught my eye are Imperial Fists and Blood Angels for the Imperials, and Iron Warriors for Chaos. I will probably get Betrayal at Calth and paint them as Imperial Fists. However, I was wondering if I could/should use them as a 40k army as well as a 30k. Also, thank you for indulging me and my noob-ish questions.

40K loyalists to 30K loyalists presents some increased problems vs traitors as the CSM units are structurally more similar in many ways.
That saud, a Pride of the Legion 30k army (elite heavy) can work.
Just started a 5000pt 30k/40k army, Wordbearers PotL/Slaaneshi Wordbearers (because who doesn't love I5 marines with fearless anf FnP?) Using 2x B@C and it works very well.

[acprince]

I am a big fan of the dark vengeance box, I bought 4 of them when they came out. The dark angels come with 3 bikes, 5 terminators, a ten man tactical squad, Chaos has a hell brute two units of ten cultists and a group of chosen, there are HQ units for both too. with access to some cheap bits from my local hobby store sidewalk sale, I found it easy enough to convert the entire set into one army. I can run them as chaos, standard codex marines with allied imperial guard, or dark angels with allied imperial guard. if you want to do this with only one box, I recommend you trade your 6 chosen for 6 or more cultists, they are beautiful models, but they are kitted out in possibly the worst options possible, and would be hard to pass off as imperials. the hell brute is basically a dreadnaught. besides the chosen the only other model I don't field all the time is my chaos lord, who I made into a custom typhus so that I could make the cultists into fearless zombies when I want to.

[Path Walker]

Don't let those posters before put you off Chaos!

If you want to player Chaos, play Chaos! They have a strong theme, loads of room for cool paint jobs and conversions and some really fun rules.
The naysayers are people who never play but post a lot online bout how bad they think the game is, truth is, who you play with matters more than how "strong" people online think your army is. If you play with people you know and like and you all trust each other to care more about everyone having a good time than who will win all the games then, with that attitude, a ton of missions and a cool idea for a story, you're going to have a good time no matter what.

As for Imperial Fists from your Betrayal at Calth set

For the 30 Power Armoured duders, set them up as three 10 man Veteran Legion Tactical Squads for Horus Heresy, ie; With a Special Weapon (flamer, plasma or Melta, might as well go for one of each ) and Missile Launcher/ Heavy Bolter, then they are usable for both 40k and 30k

The Terminators would have to have all Combi-Bolters/Power fists(option of a Sword for the Sarge because it looks cool) or all Lightning Claws to be a usable unit in 40k, either as a Terminator Squad or a Terminator Assault Squad

Dreadnought can be used as a Contemptor in both, if you have the rules or just as a standard Dreadnought in 40K, both the weapon options are usable for both games.

The Praetor is a Captain in Terminator Armour, with a chainfist and a combi-melta

Chaplain - These are really different in purpose in 30k and 40k, in the Heresy they were supposedly used to keep watch for pskyers and librarians using their banned powers as well as to be political officers in the Legions but a lot were either Word Bearers seconded to the Legions or subverted to their cause and would set up similar structures to the clandestine warrior lodges and cement support for the Warmaster.

So, while Imperial Fists can use a Chaplain in the Hours Heresy, the question is would they? I personally don't think the Imperial Fists, especially once the Heresy is revealed, would make much use of the tool Lorgar used to infiltrate the Legions, so he might be best painted black and used only for 40k games, you're not missing out on much by leaving a Chaplain at home for 30K.

Anyway, with 3 Veteran Tactical Squads, a Terminator Squad, Contemptor and a Praetor, you have a solid base for a Horus Heresy army. 3 Tactical Squads, Terminators, Dreadnought a Captain and a Chaplain isn't too bad a start for 40k either!

[son_of_volmer]

I've been a long time collector of Blood Angels, and for the longest time I was not sure if I wanted to use Betrayal at Calth to amplify my collection... then I realized, the blood Angel scheme I have is basically the Word Bearer Scheme, and then it was set. I can flip flop between Traitor and Loyalist! I mean a red marine is a red marine.

If red isn't your colour you could also flip flop between Ultramarine and Alpha Legion, they're both blue, I feel in 40k Alpha Legion are not so overly corrupted anyways.

I think a lot of people do this straddling thing. I've heard of armies that straddle Vanilla Marine, Blood angels and Thousand Suns, with a red Blood Ravens theme, hell, it can even dip over to Dark Angel Raven guard.

[Grand Master Raziel]

Most people have more experience with the tournament environment than me, so I'll bow to their expertise in the competitive meta.

However, try playing with a 40% Troops requirement. It smooths out the differences between good dexes and mediocre ones.

Chaos does have a FEW advantages over loyalists.

non-named ML 3 psykers

Cheap Cultists to use as fodder/bubble wrap

Optional bolter/bolt pistol/ccw combo

Squads that can take 2 special weapons i/o a special and a heavy

Transports that don't lose transport capacity or fire points when armed with decent weapons (Rhinos with havoc launchers as opposed to Razorbacks)

Obliterators

Just off the top of my head.