Here is a fabulous video comparing the top three interactive whiteboard apps:
Video

1. Educreations
2. Show Me
3. Screen Chomp

All of them are FREE but ultimately you need to play with them and decide which one you prefer using.

A brief comparison (covered in the video link above) follows:

Video Creation:
Educreations allows you to have multiple pages but does not allow you to erase (you can use the undo button).
In Screen Chomp you cannot insert a picture once you start recording
In Show Me you can pause and insert.
Educreations pauses automatically when you insert text or a picture.

Video Sharing Comparison:
Screen Chomp has no 'student' accounts - they don't vary from the teacher account.
Show Me allows you to register students individually where they can login and upload videos.
Educreations students can create their own accounts and teachers can create courses with registration numbers which students can belong to - students will need to be invited to them. In Educreations students cannot share created videos, they can only view them.

Community comparison:
Show Me has a comprehensive community.
Screen Chomp has no community at present.
Educreations has a community built into the app where you can view promoted videos.

Ease of use and features (summing up):
Screen Chomp is the easiest to use but only allows one slide with no ability to pause and add pictures
Show Me is simple and allows sharing of student videos within a community
Educreations has the typing text and multiple slide feature but does not allow students to share videos

A paid and comprehensive alternative is Explain Everything which you can read up about here.