A bit tough to say, since there are a few details still missing. The wiki doesn't say how much the medium +atk effect is (the small one starts at +2% and maxes out at +3%). I don't think it's also specified whether accessory bonuses get thrown onto assault like with many other things, or if they get to be their own multiplier. The latter would be better than the former.
We do know stats at least; SR accessories max out in the ~700-800 range. SSR accessories max out in the 1080-1300 range (the ones you can exchange for cap at 1080 while the ones that drop go from 1160 to 1300).
The comparison is basically, do the low to mid 3 digits flat increase in raw atk beat the low to maybe mid single digit percentage increase. The lower your atk is, the more it favours the flat increase. But if it turns out that the +atk effect on accessories get added to assault instead, then the actual percentage gain will be a lot smaller than it looks. Which would mean that the high raw atk from SSR accessories would continue to be favoured.