A balance of both. It's not too good to have Adele's sales but a 20 on Metacritic, and it's not that good to have a perfect score critically but only sell 50k albums.
Although, I guess sales are more important since they indicate the GP's view of the artist's work.
I prefer critical success for something the artist created themselves, that in the long run leads to artists having impact.
Commercial success by people who sing songs written by others like One Direction/Rihanna are forgotten much easier.
I'll go with Commercial though since I could care less what a bunch of stuffy people rate an album when 90% of the time they judge based on the the person and not the music
Commercial Success for pop artists, because critical acclaim does not pay the bills. If you're not commercially successful you will get dropped and make less music.
Critical success for artists that are independent.
Rihanna already has both with her past and especially her current era. But since she's the biggest Pop star at the moment, she has more than enough commercial success. Might as well go for some more critical acclaim.
Acclaim. Look at Fiona Apple pulling low numbers, yet receiving so much acclaim. She is more respected than any major selling artist, and that's a fact.