I want to back up my chart plotter which uses Navionics (which, by the way, I have found surprisingly accurate). Most people I have talked to have an iPad (3G, GPS) and have the Navionics App which it is said works well. Clearly there are other devices on the market; i know Android also has a Navionics App.

Does anyone have knowledge enough to compare and contrast the two? Which should I buy?

David,
The accuracy of the GPS on the iPad is excellent and it works perfectly well below deck too without an external antenna. Several different navigation apps can be used on the iPad. I have tried Imray 's and Memory Map but prefer Navionics. It is at least as accurate as our Raymarine / Navionics plotter and has the additional advantage that the charts themselves are more up to date and allow access to the community layer where users upload their own nav informartion to share with others. I have no experience of Android but assume that the Apps Run just as well.
Simon