IMO Hypancistrus Spp. would be a better option for a discus tank, they'll do far better cleaning up the let over high protein food from the discus, and don't get anywhere near as big (or messy) as a Panaque.
Besides the size difference(which I would take with a grain of salt as PC won't change a maximum size unless it's extremely well documented) they look very different as adults.
As juveniles 191 has "a windowed or clear caudal fin and white tips to the fins" as well as the dull eyes that they get their name from. As adults they lose their intricate pattern and become gray with longitudal stripes. Check the image of the adult and you'll see what I mean.
As an adult 190's pattern gets more intricate and in my opinion is the more attractive of the two. Again just check the adult pics of both and you'll see how different they become.
I'd also be hesitant about adding them to a discus tank due to their high waste output and the discus need for good water quality. That plus some pleco's have been known to suck on fish and a discus is a slow moving target. If you do try though make sure you've got wood for the panaques as it's intergral to their survival and is an often overlooked thing that isn't mentioned to novices who wish to keep them.
+1 with the last paragraph. Nobody "needs" a pleco, if you want some interesting bottom dwellers to keep with your discus then there are certainly many potential species out there, but don't think you're getting some magic cleaner fish that eats poop and craps out clean water...
IMO Hypancistrus Spp. would be a better option for a discus tank, they'll do far better cleaning up the let over high protein food from the discus, and don't get anywhere near as big (or messy) as a Panaque.