[MidnightSun]

White Scars - they have the best bikes, and bikes have speed to close the 24" gap at which Necrons will take you apart, and the high density of Meltaguns you need to deal with the extraordinarily resilient Necron armour (since the Annihilation Barge is one of the most efficient tanks in the game, it's unusual to find a Necron list without at least one and often two) and are a lot more resilient than MEQs against most Necron firepower (while you should never expect anything to last long against Necron shooting - if you leave it sat at 24", it's pretty much dead - and Annihilation Barges don't care, a lot of Necron fire is torrent Str4/5, which is far less effective against T5 than T4).

I'd advise Drop Pods for either army though - disabling their vehicles is a huge factor in being able to fight Necrons without it turning into a war of attrition (which they most assuredly will win), and Drop Melta is the best way of doing that (although perhaps less necessary with Khan - you are bringing Khan, of course - as 12" Scout, 12" move Meltaguns are very capable on their own).

Don't forget to pack your Melta Bombs - they're over twice as effective as Meltaguns, as Necrons can't simply Jink and ignore 50% of your firepower (and believe me they will Jink - Necrons don't really lose all that much from Jinking unless it's a Doomsday Ark, and really, when was the last time you saw one of those?).

 

[MidnightSun]

I have also been looking at CSM as another alternative. How would they fare do you think against necrons?

My only hesitation about the white scars is that they are effectively a bike army with more or less one tactic. A very strong and effective tactic I admit, but they are a bit one dimensional.

CSM can bring a pretty horrifying amount of anti-heavy infantry to the field, and Nurgle Spawn on a table with decent cover are a total pain in the ass for Necrons, but if you're just looking at raw ability to do well against Necrons I doubt Chaos would be appreciable better than White Scars. They can certainly win, but I don't think anyone would say that CSM are 'better' than White Scars. Basically, if you really like CSM, you won't lose horribly every game, but if you have no greater attraction to White Scars or to CSM and just care about the wins, White Scars would be a better choice.