I think the answer depends on your strengths itself. I tried both of them when I started building cross platform app.
Smart GWT Mobile
+ve: Easy to start working with Large number of ready to use components.
-ve : Support is an issue. And if there is some thing the framework didn't support, it was hard to get it done. The Smart GWT performed best when the server side also used Smart GWT. Even though Smart GWT mobile is free, I was not comfortable since they may tie it up to server similar to Smart GWT some time in future.
mgwt
+ve:
of course, open source. good support on google forums. Normally the questions are answered on mgwt forums and occasionally Daniel Kurka also jumps. Daniel is creator of mgwt and he is also a member of GWT steering committee. The themes in mgwt are good. The L & F just matched the ios or android platform. I can extend it easily if some component is not available or if I didnt like any thing.
-ve: limited number of components. It takes time to set up and start using. Slow updates.
Finally I went with mgwt since I have a team which can open up the code and fix it, if they see any issues.