Mmm... both eras were HUGE but let's see:
Album sales: 30M >>>> 27M (WW)
Single wise: I'm sure RIDT and
maybe SOY were bigger than BOMT
Critical Acclaim: Adele won like 6 grammy awards and Britney won 0 i think
Impact wise: I'm sure both eras had similar impact at their times, Britney with teen acts from 1999- to present and for Adele, due to 21's success, pop girls have to go back "normal" to have some success again and it's not neccessary to use gimmicks for now (Bangerz was kinda the exception but still), we should wait, only time will tell
Britney may have impact only, but Adele wins everything else so my vote goes to 21, easily
I could also put touring in this comparision but i'm not sure of their tour grosses so...