For read-heavy applications that don’t involve components external to Apps Script accessing the cache and don’t exceed the CacheService limits, use CacheService. For write-heavy applications or for when some external parts require access to the same cache, use Spreadsheet App.
An interesting report from Ignacio Lopezosa Serrano on the relative performance of CacheService and SpreadsheetApp for reading/writing data with Google Apps Script. Some surprising results and something I think to be kept in mind is how these tests compare to ‘real world’ conditions. As also pointed out in the post there are some service limitations of CacheService to keep in mind particularly around storage size limits.
Source: The use of Caches in Google Apps Script
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.