Full Idea
The extensional view of sets is preferable because it is simpler, clearer, and more convenient, because it individuates uniquely, and because it can simulate intensional notions when the need arises.
Clarification
And example of an 'intensional' notion might be a property
Gist of Idea
Extensional sets are clearer, simpler, unique and expressive
Source
Penelope Maddy (Believing the Axioms I [1988], §1.1)
Book Reference
-: 'Journal of Symbolic Logic' [-], p.484
A Reaction
[She cites Fraenkel, Bar-Hillet and Levy for this] The difficulty seems to be whether the extensional notion captures our ordinary intuitive notion of what constitutes a group of things, since that needs flexible size and some sort of unity.