lets just compare the success of 2 of the biggest eras we had recently.
Consider the impact, iconic moments, worldwide success, tours , records snatched , etc
If by The Fame Monster we mean the album+EP, then I guess the singles sales, music videos, tour and overall impact might give Gaga a little edge, though 21's album sales remain unmatched.
The Fame Monster by far. I mean 21 wins in sales but that's it. Impact goes to Gaga. Plus during TFM she was EVERYWHERE whether she wanted to be or not (which lets be honest she did want to be ).. besides radio and people's iPods, Adele was never around