What is the difference between Picker and MassivePicker?

Contents	Picker	MassivePicker
Performance (CPU utilization)	It is good if the number of items is small. If there are more items (more than a few dozens) performance will be reduced	Performance is constant regardless of the number of items.
Handling of items	One item is synchronized with one GameObject.	Managed by index. GameObject is recycled.
Item content	It can be freely set to the GameObject of the item.	Set by SetItemContents() of a class that inherits from MassivePickerItem.
Item size	Optional. Set by the item’s UnityEngine.RectTransform component.	Fixed size. Set by MassivePickerScrollRect.itemSize property.

Which should I use?

I recommend MassivePicker if the following conditions are true.

No problem with fixed item size
There are many items (more than 30 to 50)
You can set the item content from script.

This can be achieved either by the fact that the content of the item is simple (eg one Text component)

or by using a lot of effort to write scripts.

I recommend Picker if the following conditions apply.