I am going to respond to all three questions posed by the OP bellow:
1) The first picture depicts a Odama Rasengan, while the second picture depicts the strength of one hundred technique.
I would much rather have strength of one hundred, since with comparable chakra ( obv, a uber giant raseng is more powerful but strenght of one hundred can also be enhanced via byakuo so...) both attacks do similar damage, but the strength of one hundred is much more subtle and therefore easier to pull off.
2) Which skills would I prefer Sarada to have?
Personally I would rather have Sarada learn medical techniques and streanght of one hundred since I have seen enough rasengans from a main character for awhile. Having the main character be a medic, would be fascinating.
3) Sakura vs Naruto as a teacher
Naruto is a infinitely better teacher than Sakura by feats. Sakura has no feats of teaching anyone anything,- Naruto mentored Konohamoru, and ( probably Bolt)- and Konohamoru is now a Jounin and Bolt already knows the Shadow Clone Justu.
Think about it this way, would you rather be mentored by the strongest Kage in history, or a top notch medical ninja? Truthfully, its a little more complicated then this, it would be beneficial to choose a teacher that is a master of your own personal skills. If you wanted to be a medic, Sakura would be a better teacher, if you want to be a leader or a combat ninja then Naruto would be better.
Now, in regards to Sarada specifically - I don't really see how Naruto or Sakura would be good teachers. Sasuke would most likely be the best teacher for Sarada, since he can teach her how to use the sharingan + she most likely has fire style, so he can teach her that.
However, if we are only comparing Naruto with Sakura for potential teachers for Sarada, - Then I don't see what Naruto could teach Sarada besides leadership skills/talk no justu. Sarada isnot known to have massive chakra- so no sage mode and grand ninjustu ( as of now), + Naruto doesnot know how to use a sharingan, - meaning that Since Naruto's skills cannot really help Sarada, then Sakura would be preferred in this specefic scenario.
Note: With the Gaiden, their has been a massive time-skip and nobody knows what various characters have learned or the type of people they have become. - Their isnot enough information to accurately respond to the OP's questions. My answers above are based off of what little we know and speculation.