Recently a comparison between Adafruit GFX lib and U8g2 became available on the WWW:
Although U8g2 is not better than Adafruit GFX Lib according to this article, it nicely describes differences, problems and features of both libraries.
So, if you are in doubt which OLED library to use, then maybe this article can put some light on this question.
My comment on the results of the comparison: Performance tests reveal that u8g2 is slower than Adafruit GFX lib. As author of u8g2 lib, this is my remark: (A) The comparison was only done for text. Other graphics output was not considered (and is hopefully as fast as Adafruit GFX lib) (B) The reson for the slower text output of u8g2 is mainly due to the compressed font format. The benefit is, that U8g2 fonts are much smaller than a comparable Adafruit font (leaving more Flash memory for your code). The drawback indeed is a slightly slower text output.