There's absolutely no specification if there is a separate SDK for premium or if premium key allows some additional Google Maps functions for the iOS part.
Imho same sdk, different api keys.
edit: It`s same SDK and you are spending your free limit (source):
The 100,000 daily free requests are shared across all Maps JavaScript API client-side services and Google Maps APIs web services—all requests are subtracted from the same pool of 100,000 free daily requests. Any additional requests are applied against the total number of Maps APIs Credits you purchased for your Premium Plan. Your free daily request pool is reset at 12:00 am PST. (source)
If you continuously reach your limit you will have to upgrade your plan to premium - but you will still have the same SDK.