There is a decent amount of difference. The obvious one is wattage. The 2:90 is simul-class so it runs in class A/B and class A where the 2:100 is just class A/B.
The 2:90 also has a half drive mode, deep mode, and modern mode.
For the most part the 2:90 is a more flexible power amp where the 2:100 is basically all about power.
ok so I'll keep my 2.90 ! I've got enough power with it..... the only trouble I've with it is a little hum when the deep function is on but nothing more.