Male nyala have a mane and a beard that runs their entire underside but an otherwise rather sleek coat, whereas male sitatunga are just generally shaggy all over but with no distinct mane or beard. Male nyala also have reddish brown lower legs, while in sitatunga the lower legs are the same color as the body. Sitatunga also often have a horizontal stripe on their lower abdomen that nyala do not have. Those are probably the most obvious differences from a distance. There are other anatomical differences that you pick up on once you’re more familiar with them or if you’re closer, like the shape of their faces, the very exaggerated hooves of the sitatunga, and that male nyala tend to be more slate grey while male sitatunga tend to be a softer, warmer, brownish grey. Those are all more subtle differences, though.
@Kudu21 That's a huge help, thanks! Saving all of that as a note Since I usually only see females, I think of sitatunga as fuzzier than nyala, but didn't know beyond that or for males.
Sorry some of my identifications are off. This zoo had a good unglate collection which is part of why I liked it. I saw my first blackbuck, impala, and white-tailed gnu which I thought was great. I'd rank this zoo in the top five I've visited.