OK--based on what you have all said I believe you are more informed than I about the technology so I am going to explain to you what I need and maybe you can suggest appropriate unit:
I need a portable chart plotter for three reasons:
1) I do not own a cruising sailboat;
2) I charter sailboats both in the US, Caribbean, Mediterranean and soon-to-be other places so I would like to bring this unit with me (in addition to my paper charts);
3) I am a sailing instructor and sometime use the device to teach the basic pros and cons for GPS Chart Plotters versus traditional piloting.
Which unit do you guys recommend?