Tough decision. You got (depending on how long ago you paid $440) the 28 for a steal and in your posts you note how much you really like it but barely mention the Service Six, which functionally is the same gun as a 681. (Ok, the 681 is more like a GP100 but you get the point...)
However, you seem to have a problem with getting rid of your several "plastic auto loaders". With the choice of keeping a nice steel revolver, even against several of those, to me the choice would be clear, especially if the semi-autos are current production and cheap. Those are likely easily replaceable. If you are not prone to regretting selling guns just get rid of the nice, out of production, hard to find in that condition steel revolvers that you like the least and hang onto the plastic.
Good luck in your decision.