Anecdotally, the Arturia controllers feel better to me in terms of build quality. Granted I haven't owned an M-Audio controller in well over a decade, so take that with a grain of salt.
In terms of what the knobs and pads are for, you'd be able to assign those to parameters of the Slim Phatty using the appropriate midi CC#s (here's a forum post where a user mapped them out). So you'd be able to assign one of the knobs to the filter cutoff for example, and one of the pads to switch the octave of oscillator #1, etc. Possibly useful if the Phatty is racked and you don't want to walk over to it while making a patch, or if you want to manipulate parameters while playing live.
As far as I can tell, you're not really shopping for a feature set beyond what any standard MIDI controller would offer, so you're likely safe buying any well-built controller until your needs change. You might be able to get away with something even more basic without extra knobs/pads like an Arturia Keystep. I'd suggest heading over to your local instrument shop and demoing a few controllers and see if something like the width of the keys, build quality, etc. matter to you. Good luck!