I didn't follow Trump's issues closely at the time, but, from what I know, his behaviour seems worse than Franken's. On the other hand it's now largely moot, since all the accusations were in the public domain and the American people still made the decision to elect him president.
As far as Franken goes, if nothing more serious emerges, I don't think he's obliged to quit, and probably won't. His electorate will have their say when he comes up for re-election.
For me the really serious cases involve abuse of power, and that's the saving grace for Franken - none of the women were his subordinates and didn't have to put up with any crap. Plus he seems to have behaved appropriately with the women who worked for and with him.
Even though it hasn't gotten as much publicity, the accusations against Disney's John Lasseter are a good example of serious wrongdoing. The story of the woman in a meeting sitting beside him twisted into a defensive posture with her arm across her thigh to prevent his hand 'traveling upwards' from her knee, is shocking. No one should have to put up with shit like that at work.