They're both initially a bit shy with new people but open up pretty fast. At that point, Avery is kind of a clown - funny faces, toddle jokes, etc. Quinn is much more reserved and absolutely an introvert.
The difference is apparently in their physical development too. Avery was always a few weeks ahead of Quinn in gross motor skills. We used to attribute it to Quinn's IUGR, but it's possible some - especially the later milestones like walking - is just her. At nearly three, Avery flies on a tricycle. Quinn can sort of kind of pedal a revolution or two but has no interest and minimal skill. But, she writes beautiful Qs and can do zippers including starting them and some buttons.