<i>Ancistrus dolichopterus</i> is generally much darker in color, and has more dorsal-fin rays (9 vs. 8). It's also supposed to have a markedly longer lobe of the caudal fin.
It's next to impossible to tell which is which just by looking at color pattern alone, as several species of <i>Ancistrus</i> are known to have a spotted pattern like that.
Fin ray numbers are a lot less variable, and so is more reliably used to identify species.