Make sure you choose the right organiser for your needs. These questions might help you.

What is the data capacity you are after for the electronic organiser?

Over 5MB - ZQ590A has 8MB Flash Memory
Over 1MB - ZQ290H has 1.5MB Flash Memory
Around 1MB - ZQ20H has 1MB Flash Memory

What are the display specifications for these organisers?

ZQ590A - 80 x 111 dots high resolution display
ZQ290H - 14 column x 5 line display
ZQ20H - 12 column x 3 line display

3. Are you interested in having diet function with built–in pedometer to keep track of calories burned by walking?

Both ZQ290H & ZQ20H have this function.

4. Is it correct that all the Sharp organisers can synchronise with Microsoft Outlook?

Yes

5. Is there any spell-check, translation functions in any of these models?

ZQ590A - Spell checker & English/ Spanish translator
ZQ290H - Spell checker
ZQ20H - Spell checker