They seem very similar to me, so much so that one wonders why two classes so alike were deemed necessary
Aside the two being designed from the same CME, there are some differences in terms of operations. The Jubilee (Class 5XP by LMS, later 6P by BR) is for express passengers that operates mostly from St. Pancras in London, while the Black 5s (Class 5F5P/5MT) were built for mix-traffic work that can do anything from anywhere on the Midland Region lines at the time. Not to mention, there were a lot of Black 5s (well over 840 units), compare to the Jubs (191 units) that were made. But I doubt that makes much of a difference in terms of quantity. Other than that, not a whole lot of difference by looks, but that's how Stanier built them by design.
As above said, the two were designed for slightly different purposes. The Black Fives were very much a development of the GWR Hall Class, being mixed traffic while the Jubilees were designed more for intermediate express passenger trains, being designed as a Stanier equivalent to Fowler's Patriot Class. One of their first duties was being allocated to Euston to Birmingham services alongside the Patriot Class. They also worked portions of the Royal Scot train from Carlisle to Glasgow. They monopolised expresses on the Midland Mainline out of St Pancras, especially in BR days, which is why I'm a bit annoyed they were chosen for Spirit of Steam. They weren't known for working that route at that time, and principal Euston to Liverpool expresses would have been in the hands of Royal Scot 4-6-0s and the Pacifics, but I digress. Some key differencs are that the Jubilees have larger driving wheels, and a slightly higher tractive effort. The Jubilees had three cylinders, while the Black Fives only had two. Due to these differences Jubilees were more route restricted so the Black Fives could work on smaller lines the Jubilees couldn't. ALL the Jubilees were named, but a VERY small amount of Black Fives were named. This is the first thing I look for when trying to tell them apart. The Jubilees have larger and longer boilers, and thanks to their larger driving wheels they have splashers along the running plate, which is another way to differentiate them. There are a lot more differences I'm sure, but I'm not really one for intricate technical details of steam locomotives. There are more differences than meets the eye, enough that they are in separate power classes (6P for the Jubilees, 5MT for the Black Fives, hence the nickname).
