Both are bad choices unless you want to do all of the work yourself. If you do then both have very limited program memory (1K words for the PIC16F676). They are also very old.
Look at something like the PIC18F26K83 which has a CWG module that does what you want.
I'm not sure what you mean by 'changing the duty cycle through ADC' but that MCU has an ADC module as well. It also has a lot more FLASH memory for your code (although the hardware will do pretty much all you want in terms of the critical dead band work).
There may well be other MCUs - this was just the first one I saw that had a suitable PWM configuration.
Susan