Hmmm... I'd say that Selena has better tracks than Ariana. By this I mean that she has better highlights in her catalog (Stars Dance (song, the album is atrocious), Perfect, Same Old Love, Hands To Myself, Sober)
But as a whole, Ariana has better albums and better music overall. And yes, she also has amazing tracks but in my opinion, I like those mentioned above more than any Ariana song. But it's just my opinion.
Idk if this makes any sense.