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?

I too had the initial Navionics application on my ipod touch to see what it was like. However I baulked at the cost of $60 coupled with poor reviews when my nearest and dearest gave me an iPad for my birthday.

We have multiple levels of redundancy for navigation.

Raymarine chart plotter at the helm with Navionics.
Macbook Air running Polar view coupled to the independent GPS in our AIS
PC Laptop with OPENCPN (free) if the Macbook goes down.
We have our Android phones with gps and applications that will give us a readout of position and now, our iPad with position and compass course, speed and the like as well.

But, I hear you say, what about the charts. We take a printout of the appropriate electronic charts at appropriate scales for our route from the PC before we go.