So our certified pop legend girls all have one or severals geniuely great performance(s) where they clearly tried to raise the bar wether by great vocals/choreo/production value/controversy
Britney has all her VMAS and others
Beyoncé has the Superbowl, SL VMA performance and others
Rih is slightly lacking is that area but does have Grammy 2012
Katy is lacking too but does have the Superbowl
Gaga has Paparazzi VMA and is overall a very consistent performer and always tries to be innovative with her stage or outfits, she also does good choreo most of the time

Now the newer girls (Ari, Selena, Demi...) always seems to play it very safe and have 0 (to me) memorable performances.
The only relatively new girl that's a great performer is Nicki Minaj who always has something wether it's choreo (although minimal), controversy she always goes for it in her performances, that Grammy perf is an example

What's your thoughts on this ATRL ?

Tit for Tat

Rihanna attempting to sing live at the BBMAs and succeeding.

Selena Gomez seems to be the most ambitious of the 2 you posted, but still not very good live.

While I wouldn't describe any of Ariana's performances as iconic, I think all three of her AMA performances are somewhat memorable and ambitious - at least more so than anything Demi or Selena have done.