Sorry, I've been away for a couple of days ...
The SPn has a battery check button, and the cosmetics around the VF are different, i.e, the black trim. Other than that, it's the same as the SP ...unless they changed the leather, but it doesn't look like it from Shadow's foto.
I prefer the SP to all other Oly RF's I've had so far, except for the XA in certain circumstances. I like the quieter shutter of the LC better, but the handling of the LC and the viewfinder are not as good as the SP, IMO. I have never used an RD. I haven't used my RC very much, so can't really compare it except that in manual mode the controls are a bit small and cramped for me. But as a small shooter with a good RF, it's a great little camera.
I would love to get a good Ace kit some day, but the Ace in good condition with a full set of lenses are a bit dear, especially since I don't expect that the lenses are that great (1958 vintage, afterall), though I could be surprised.