The *rules* favor
CSM? Which ones?
I will allow that their codex gives them better equipped troops, with that extra
ccw in h2h and 2 special weapons per squad. And
PM & Khorne are superior troops choices. And the Daemon Prince
HQs are just hella great. Oblits!

But,
CSM choices are not as plentiful as Codex:
SM. They have many more cool things than
CSM do:
Librarians
Drop Pods (use a dreadnought
DP against your
CSM opponent, maybe the Ironclad as Drop Podding troops in is a bad idea)
ThunderFire Cannons
LR Redeemer
Speeders
TH/
SS termies
If you are closing and going toe-to-toe with
CSM with your tacticals, then you'll lose, as they are assaultier and tougher:
KhorneBs - a lot more attacks, Furious Charge, Fearless
Plague Marines - xtra h2h, T5,
FNP, Fearless
Play a shootier list, try the Dreadnought DropPod if you have it. Do ya have a
LRR?
Use LasCannons or
MMs to deal with his Obliterators ... or that Dreadnought DropPod.
Also,
CSM lack in good Fast Attack and their elites are okay.
SM have many more, and better elite choices, especially
TH/
SS termies.
If you really want to despair about codex differences, compare
SW,
BA and
SM.