Unimap is a very rudimentary map. If you go with Navionics Plus you MUST use the download product that stays under the 2GB download limit. You CANNOT use the 6GB preloaded product for USA and Canada. That means you have no choice but to use the "World Regions" download product if you pick Nav Plus.
I use that with my Helix 5. It works very well. Note that the Helix 5 can display both the nautical charts as well as the Sonar charts.
By the way, the 2GB limit is not much of a problem. I have all of Washington, Oregon, Idaho, western Montana, and southern BC on mine in just 1.5GB of space. The map loading software does a great job of showing you how much memory your selection takes up.
See the compatibility chart on the Navionics website for details.