One thing I can say about Yoona is that she is consistent, her movements are on point and she never really looks sloppy. That isn't to say Sooyoung looks sloopy she has great stage presence, however she has more lazy days than Yoona lately and she is consistent with her dancing as Yoona.
I am basing this off the latest SMTOWN performances

Her dancing is more fierce and she executes the dance moves in a better tone than Sooyoung I think

Yoona is more precise with her dance moves; it's rare to see her mess up on any performance.
Sooyoung just has more stage presence to me, so that's Yoona's only setback.

For me, I picked Yoongie because I can see that she is very energetic when it comes to dance practices and live performances, i like her dance moves it is very gracefull and clean. But when it comes to confidence i'll choose Sooyoung but she's also good.

Honestly, these two are pretty much tied for me.
Yoona: Very precise and has sharp moves. Lacks expression and stage presence.
Sooyoung: Has stage presence. For me, she only has a sexy expression when dancing.

According to Sm,its Sooyoung.......
and for me too

Yoong

They have a near level of skill, but I get bored looking at Yoona. Sooyoung has infinitely more stage presence. And she has that sexy halo.

My reasons are pretty much like snsdlover4ever's.

I like her moves better. Sooyoung is very good too, energetic and feverish.

Everyone knows it's Yoona ^^

Both are good.