Full disclosure: I work on Material Components for the web, so my opinion may be a bit biased :)
TL;DR use whichever library helps you build your UI in the most efficient way possible, or use them side-by-side. Check out our angular2 integration example angular-mdc-web to see how to wrap a MDC-Web component using ng2.
The goal of Material Components for the web (abbrev. MDC-Web), as has been alluded to, is to create a canonical implementation of Material Design components for the entire web platform. Angular/material2 is an excellent library - many of us on the Material Design team, myself included, have contributed to it - but it is exclusionary to non-angular2 applications. Additionally, non-google libraries and frameworks such as React, Aurelia, Vue, and others have no official solution to suit their needs.
That being said, Angular2 falls under our scope of the "entire web platform", and we definitely want to support it because of that. At this stage in both projects, we may have some components you need that angular/material2 does not, or vice versa. I'd encourage you to use whichever library helps you accomplish your goals in the best way possible. That could be angular/material2, it could be MDC-Web, or quite honestly it could be both. For example, you could use our select component along side material2's components, and things should work just fine. As mentioned in the TL;DR,angular-mdc-web is a fully-featured library that wraps MDC-Web in an idiomatic angular2+ API.
On a final note, MDC-Web is the only library that is developed by the Material Design team itself. Because of this, we are able collaborate on and iterate with the same people who author the Material Design spec.
