PWM is one step away from real time. S-D takes a lot
of cycles to do its thing. That's OK if you've got a high
enough clock or a low enough must-have sample rate.
But at your low-end microcontroller clock rate a S-D
ADC would be hard pressed to keep up with audio.
PWM also needs only a counter and compare, or two
counters, no thinking whatsoever. S-D gotta mash,
mash, mash and then maybe you get a number.
Dithering a PWM signal (for reasons I guess) only
makes it all more demanding, more thrash / switching
losses, constraints on minimim on / off time that a
PWM would likely never encounter (if sensibly designed).
Ask yourself what the purported benefit of S-D has to
do with your project's specific care-abouts. Why other
people prefer what they prefer? Doesn't much matter
unless they're doing what you're doing.