If you look at U100 and U90 when they're closed, you wouldn't notice any difference. But, just one look at screen and the essential difference is revealed - U90 has 8.9-inch screen which is for 1.1-inch smaller than display on earlier introduced model. As for interior is concerned, there are no bigger differences and even BIOS is the same for both models. Quite simple, it wasn't economical to build completely new model so MSI adapted existing one with few changes in hardware components and software setup.