The C-series drills have excellent ergonomics. Years ago I owned a C12, and just bought a C15 a few days ago. The C shape allows you to hold the drill in a manner where the motor, driveshaft and chuck are inline with your wrist and forearm, my hands are large enough that I use my pinky and ring fingers to actuate the trigger.
Everybody is going to have their own preference, but I'd certainly cast a vote for you to go with the C- series.
I don't think the 4.2 packs are any larger than the 3.0.
Now, if only Festool released an impact driver in the C format!! [big grin]