Doing a search in German records at FamilySearch for both Noak and Noack exact searched give approximately 3,600 hits for Noak and 59,000 for Noack.
Using the site http://www.kartezumnamen.eu/en/ Noak shows in small numbers all over Germany. Noack, on the other hand shows up in large numbers in Brandenburg (centered on Berlin), with significant numbers in Saxony and in Rheinland-Westfalia.
From this I would conclude that Noack is the most common German version.