The Garmin includes the Azores with Europe for that price, whereas Europe is another $75 on navionics.

I had a chance to compare the Navionics and Garmin iOS apps side by side in the Caribbean a few weeks ago. Navionics is easier to use and seems to have a few more useful features as a standalone app. If you also own a Garmin CP that talks with the app, then the Garmin has added integration features that tip the scales the other way.