Ideally we want both, but diversity is more important, because it's a necessary prerequisite for choice. You can have real diversity without choice[0], but you can't have real choice without diversity[1].

[0] Imagine that everyone was mandated to use one of five different browser engines, such that each had exactly 20% market share.

[1] Imagine that you are free to choose between five different browser engines but everyone else picked #3 and websites are only ever tested on #3 and all the others don't work so well.