Multitasking is a matter of code. You set a timer to hit an interrupt every when, and mean while you do what you need...
Will a 20MHz to different than 16MHz as for setting the timer.... Not so much.... Does your program evolve communications?
Also not much as communication with ICs are at some certain bauds. Of course you can hit higher bauds in some application but still not so much of a talk...
Will your processor do math or so? Maybe you see a difference... Will it do heavy math??? If yes... 20MHz is better...
And tell you what... Since your project doesn't include communications or you can handle writing the communication subroutines,
overclock it...
As far as I heard and tested atmega8535 handled 22MHz and that's 135,7% of it's initial speed...
At 24 it run but became unstable with the ADC, mostly because ADC must run on certain speed spectrum and the prescaler wasn't just enough...
I bet newer cores like atmega32 or the atmega2560 (axon) would handle 22MHz nicely, still have some thoughts about 24MHz...
So considering this it comes to a point that with even newer cores like atmega168, 24MHz is a sure thing and 26MHz still to be tested...
But I haven't tested so, I can only bet...
Still you need these speeds if you have to do with derivatives and angles... But then I'm starting to feel you need a DSP and not a microprossessor...
So.... What is better at multitasking without telling us the tasks.... Is like giving a blind man a bow and a target...
Also when saying multitasking... I do point out that it can not be done with serial programming.... That is the easiest but less efficient coding method...