The Atmega ADC on the Arduino should suffice, and since DC motors are very noisy, always a good idea to do a LPF of any sort. The standalone ADC chips are very nice if you need to use more than a couple channels for sampling, and will always provide more accurate results because all it does is the conversion. The Atmega ADC is nice, but if you’re doing anything in realtime you’ll have to figure out if you need to use an interrupt system or go with the good ol' polling.
Either way should work just fine.