When you win an arena rare spin for filling up the rings, the rarity is identical.
When you win a regular spin from the deck itself, SoR might be more rare because less people have it in their decks.
I'm at work, so I can't check my account, but I know that of the 3 older shards (the ones that existed when I started) I have a full set of unupped/upped SoD/SoG but only 4 unupped SoRs and 6 upped ones. I think it's probably just because of my second point above, though.
There are certain rares I hardly ever seem to spin, though, but other people have a different set of rares they can never spin :p For example, I have only ever won ONE upped titan, but I've sold 20+ each of upped morning glory and eagle's eye. It's just bad luck if you can't find SoRs. (or in my case, upped titans)