The new one will most importantly give you a force you can use in pick up games and that will survive most short term extra changes to GW, you haven't got used to the awesomeness that was the old one so you won't feel the face slap that the new one is when held up beside it, you won't be using demons, so you'll just miss the customisation of being able to cherry pick any vet skill\USR you like (e.g. at the moment I'm remenising back to +1 cover save for a point)(and maybe for story you'll miss the cultists and warsmith rules). But as I say if you don't hold them up to the light side by side the chaos codex is ok.
Good elites (apart from possessed, even the dreadnought is almost passable) Good heavies (But you fill those slots way to fast and most things you want donw can just be done by using a sack of obliterators)
Mish mash of HQs (Basically if you want to not lose you take some form of flying demon prince. sorceror being wayy over costed, lord only doing stuff when over costed (but he can be cool to model) Named charectors ahirman kharn and abbaddon are awesome (and points /\reflect it),huron is boring and not worth it (Haven't got round to the rest)
Troops basic CSM as standalone soldier is awesome, but..you run into a massive hurdle here in that the new codex really hates infantry with heavy weapons.. so on top of the increases in las plas ( and heavy bolter
) prices, you also have to dedicate a 10+ man squad to be able to have them, which makes them real ordinance bate, so basically maybe not bother with heavy weapons.
Icons- icon of greater indecision has a place, anything goes well with termies, and slannesh and khorne go well on raptors apart from that I haven't had an inspiring time with them.
Cult troops are ok, but for each way one is good I could tell you why one is bad (e.g. 1ksons now laugh at las plas, but their squads get totally destroyed by a heavy bolter etc)
If you need more info I can link you to a few tacticas or write something a bit more in depth somewhere.(but go into a shop and read it before buying it, especially draft out how you'd use the force organisation chart and how you feel about configing your troops choice and whether the lord allows you to use him like you want, if your not using it for max plas, smurf might be more versatile for you)

For a forgeworld space marine army there are a number of traits you could use:
The one that gives you Iron Fathers (forget the name) - but give techmarines as HQ choices & terminator squad sargeants (not that effective, but definately cool!)
Cleanse & Purify
The one that gives loads of devestator squads (plasma cannon lovelyness everywhere)

If you want a heavily tied in Adeptus Mechanus / Space Marine army, I'd recommend The first 2 in the list above, or perhaps going DA so that you can pick a 'slot free' techmarine per vehicle.

I'd say that the Chaos list could work well, but you lose a lot of the cool stuff SM get - but specifically Techmarines that would be important for a forgeworld chapter.

Thinking about it though, the Thousand Sons rules could be 'fluffed up' to be experimental plasmabolt guns - the reason that they have slow & purposeful is due to the immense power needed to recharge such powerful & weapons (even psychic powers could be given a 'Adeptuis Mechanicus twist). Obliterators could be a new multi weaponed experimental armoured suit (they might be based on Tau battlesuits - the Imperium would like something similar...).

SM allied with =][= (representing AM operatives) could be fun & fluffy - especially if you get some penititent engines to represent unstable new dreadnaught designs. Even the humble DeathWatch could be used with their 'super boltguns of doom'(TM).

Really how you represent the army is up to you... As long as you can create a 'logical' and unifying theme, I doubt you'll have much trouble.

Good luck

Ljundhammer wrote:the Thousand Sons rules could be 'fluffed up' to be experimental plasmabolt guns - the reason that they have slow & purposeful is due to the immense power needed to recharge such powerful & weapons

Indeedy as chris tzeentch over in the tzeentch tactica on warseer (and on his own forum too) has been doing http://warseer.com/forums/showthread.php?t=112422

Ljundhammer wrote:SM allied with =][= (representing AM operatives) could be fun & fluffy - especially if you get some penititent engines to represent unstable new dreadnaught designs.

If you mean to take a SM army and add WH units, you couldn't take pentient engines - they're heavy support, and you cannot take WH heavy support units as allies; otherwise I'd be very tempted to take an Exorcist