Quote Originally Posted by sanahtlig View Post
The SSR accessories have much higher stat caps. The 500 or so attack gain from an SSR is much more important than 6% (3%*2) higher character attack for one character. Also, you can keep farming until you get approximately the effects you want.

Ancient accessories aren't mentioned in earlier versions of the wiki. They were likely added with rank 5 accessory quests.
At least according to the edit screen, even the raw stats of an acc are also only for the character equipping it. So 500 att will be better, but not massively better. A more interesting question would be how 4% double att chance compares to 500 att.

Realistically though, you shouldn't be spending any effort on leveling your accs until you get good ones.