They both belong to the same suckermouth catfish subfamily (Hypoptopomatinae) but they belong to different groups (tribes). So they are related but not closely. Otocinclus tend to move throughout the water column whereas pitbulls spend most of their time on the bottom. According to the experts Parotocinclus species need clear, unpolluted, oxygen rich water, and they can be tricky to keep. However, pitbull plecos seem to be an exception as they are collected in large numbers in waters polluted by human effluent, so I guess thats makes them fairly hardy. They are apparently best kept in groups.