I would suggest two mauler fiends. I ran a list with two mauler fiends and two units of five nurgle spawn, and it was awesome. I had all my infantry in rhinos and some bikes and I moved flat out on turn 1 with pretty much everything. I got in his face and overwhelmed him with all of my guys. The thing I would stress, though, is that mauler fiends shouldn't be your only CC. They move fast and can easily get ahead of the rest of your army, so you have to have a list that will keep pace with them or else they will die before they get near the enemy with no followup to make the death worthwhile.
As far as Forge fiends are concerned, they are ok, but they cost the same as two helbrutes if you pay for an upgrade, and i would rather have two helbrutes. That being said, they have their uses, but they only have BS 3, so the autocannons dont really attract me, whereas the ectoplasma cannons can be very deadly against any army with infantry. Forge fiends aren't bad, but they arent great. They are a middle of the pack kind of unit. They are fun and will scare your opponent, but they are by no means the best thing we have.
Final say, since I like cc, i would go mauler, but they are both fun and they are both useful for different things.