Ariana. Demi has already lost her voice from the screaming. No shade.
Excuse me? She's at her biggest point vocally, she can do wonders with her voice right now.
I have no idea what Demi are you talking about, but it's not Demi Lovato.
For me both are exceptional vocalists, they have their strengths and weaknesses, but Demi does something that Ariana doesn't for me, she can easily give me chills and make me feel every note she hits.
It's kind of hard to compare them because they both use their voices differently and succeed at it. They're both capable of making me feel something. Ariana has the better technique, but they're good at delivering a message to the audience
From an objective standpoint, Ariana is the better vocalist.
Ariana has a slightly larger supported range. Ariana has a larger studio range - and a much larger live range. Ariana's four octave and one note range is fully connected while Demi only achieves her four octave and one semitone range due to one studio whistle. Demi also uses tremolo (or fake, uneven vibrato) instead of real vibrato and doesn't know how to mix in her belting range. Ariana also has the better lower range (Demi becomes airy below A3 while Ariana has supported as low as F#3. Ariana has the more developed falsetto. It also doesn't help that Demi is inconsistent (see her Stone Cold performances).
Demi only trumps Ariana in her head voice (Ariana rarely utilizes head voice), less tongue/jaw tension, and less high larynx (although Demi suffers from low larynx).
Because it's not true. Ariana has a much better technique (her technique is by no means great, don't get me wrong, but still better). Demi's technique has actually gotten worse over the years. Both are probably equal in potential though.