Match the Tool to the Task
If you need to read an MGRS grid, set a waypoint and work through navigation calculations, Red Grid MGRS offers a focused app on iPhone and Android. Core navigation needs no Red Grid account.
ATAK is a broader geospatial and team-awareness platform. Its official listing includes offline and online maps, overlays, shared locations, video and extensibility. Offline mapping is not unique to Red Grid. See the official ATAK-CIV feature list for its current capabilities.
Red Grid is not a TAK client. If your organization requires TAK-server connectivity, CoT exchange or ATAK plugins, the current Red Grid release does not satisfy that requirement.