First off Zach good to see you on here! Now that it is winter and I have a little more free time I hope to frequent the forum some again. You guys did a good job describing the differences between these two but I figured I would add a little and some photos since these are some of my favorite natives to keep in aquariums...
Steelcolor... Breeding males have the pink/red nose, very enlarged rounded rear edge to dorsal fin, yellow lower fins, scales have dark edges giving body a cross hatching appearance, and nice steel blue sides with white tips to the tail. Females and non-breeding males I look for the absence of a well defined gold line down the side that is best seen looking at the fish at a somewhat downward angle or just in the water it is more obvious. Also you can use the 9-10 anal ray count but I have seen quite a few Spotfins with 9 anal rays so certainly use these in combination. Also rear edge of dorsal should be somewhat rounded and over all body should look stockier/taller with a taller caudal peduncle than that of a Spotfin.
Spotfin... Breeding males lack the pink nose, have a very straight rear edge to the dorsal fin, bright white lower fins in most populations (upper Mississippi Basin display yellow like Steelcolors most places including the area being discussed), scales have dark edges giving body a cross hatching appearance, and have steel blue sides with white tips to all fins. Females and non-breeding males have a well defined gold stripe down the side that is best seen looking at the fish at a somewhat downward angle or just in the water it is more obvious. Also most Spotfins have only 8 anal rays but this is not consistent and has caused a lot of issues with the supposed distribution of Steelcolor. Spotfins have a more elongate body over all and a narrower caudal peduncle. Even I don't get all of these right, I make sure I find an obvious male Steelcolor before I declare them definitively occurring at a site.
As an example in 2016 I found what I thought were young Steelcolor Shiners in the Little Muskingum River which there were no records of in this system other than a single specimen from Milton Trautman in 1929. This year I was back in this system with three helpers(thanks Andy, Ben, and Zach!) and we worked the area over hard until we got the definitive answer of a very nice breeding male that I will post the photo of below. We also caught about 8 other females or non-breeding males this year. This population represents the most upstream definite population (confirmed by vouchers... Trautman's 1929 fish OSUM 10870 and our 2017 fish OSUM 118147) in the Ohio River basin. There are reports and even preserved specimens from further up the main-stem Ohio but these all turned out to be miss-identified Spotfin Shiners when I investigated all available material a couple winters ago.
Breeding male Steelcolor Shiner from the Little Muskingum River August of 2017
Female Steelcolor Shiner from the Scioto River Ohio
Non-breeding male Steelcolor Shiner Scioto River Ohio
Comparison shot of two small fish Steelcolor top and Spotfin bottom showing absence/presence of the gold stripe Elk River WV fall 2017
Breeding male Spotfin Shiner from the Kokosing River Ohio
Female Spotfin Shiner from the Kokosing River Ohio