Both dramas star Jiro Wang as an arrogant popular singer that has never experienced love, only to soften as he falls for the female lead. In both dramas the female lead is shy and rather unworldly, and the hero is protective of her, although at the beginning he doesn't understand why he cares so much. Both dramas also show the two leads living in the same house, so both shows feature a lot of sweet cohabitation scenes.
However, Fabulous Boys has more characters, so it develops the side stories perhaps a bit more than ToGetHer. I also felt that the hero in ToGetHer went through more of a change than the one in Fabulous Boys, because apart from falling in love he also loses his popularity, whereas the character in Fabulous Boys never quite reached that level of vulnerability (he was more arrogant and stayed it longer).
Both are romantic, funny and light, with a little bit of drama towards the end (but not too much). Also the actor that plays the manager Jie Ge / Tony Ge is the same in both dramas :p