The decision to trade or not trade largely depends on what you intend to do with the Shopsmith or ATF55 and which machine you would get the most use and enjoyment from. I purchased a Shopsmith 500 new (1980 vintage) and owned it for about 27 years before I sold it. I also had an ATF55 that I gave to my son when I upgraded to the TS55. Strictly from a dollar value standpoint, they are probably worth about the same.
Since you don't own a Shopsmith, I will limit my comments to that machine. As with any multi-function machine, there are always trade-offs and the Shopsmith is no exception. It does several jobs okay but, none with superiority. As a table-saw, the Shopsmith is the absolute worst I have ever used or owned. The table is small and the tilts instead of the blade which is an unacceptable limitation for me. The fence is to put it politely, marginal. As a drill press, it is pretty good but, don't even think about using a hollow chisel mortiser with the earlier version of the table as there is way too much flex in the support system. As a horizontal boring machine, it is okay but nothing to write home about. As a lathe, I would classify it as marginally acceptable primarily because the tailstock has way too much flex in it. The disc sander works very good at two functions; 1) sanding and, 2) spraying dust all over the shop.
I don't regret owning the Shopsmith 500 as it gave me multiple functions at a time when I was just getting started in woodworking. I also don't regret selling it as my needs grew such that they far exceeded the capabilities of that machine.
Hope this helps.
Steve