Some components aren't present in MDL such as dialogs/modals. The scaffolding grid system looks very similar, but just from browsing around MDL, I prefer Boots

Some components aren't present in MDL such as dialogs/modals. The scaffolding grid system looks very similar, but just from browsing around MDL, I prefer Bootstraps implementation. All of MDL's classes are prefixed, so you could use both frameworks.