I grew up GC Mennonite (which has become part of Mennonite USA) and went to an MB church as an adult.
Big sweeping generalizations here, and it might be more of a local phenomenon:
Politics
MBs: Republican
Menno: Democrat, Green
Mission work
MBs: All about "saving souls" and bringing the gospels to people.
Menno: Expression of faith to help people. Will share how/why faith motivates it if asked, but not big on prostelyzing.
Peace & Justice
MBs: Really not an emphasis. In fact, the last MB church I went to had members that had no idea that it was traditionally a pacifist church. There were a number of military members.
Menno: Huge in most of the conference. In many ways, Mennonites were "hippys" (without the happy drugs) long before it was popular. Were driven from countries and persecuted because they refused to bear arms. "Non Violent Conflict Resolution" is really big and some Mennonite Colleges have majors in it.
Social Issues
MBs: Fall very much in line with mainstream evangelical and non-denominational churches IRT issues like gay marriage, women at the pulpit, etc.
Menno: Split heavily between congregations, although the conference in general "agree to disagree" between congregations. I have been a member of several congregations who had females in leadership roles (including pastors) and a member of church that openly supported gay marriage.
Relationship to Other Faiths
MBs: the church we went to for a while sent out a "prayer calendar" to pray for Muslims in war torn areas. The request was, basically, to pray that they see the error of their ways and become Christian.
Menno: the church I grew up has a "peace lantern" that is lit every week, before the service, to remind everybody to pray for peace and non-violent conflict resolution and healing for those in war torn areas.
Menno's are big on inter-faith initiatives and community groups and encourage others to seek out the similarities between faiths as a starting point to foster peaceful coexistence. MBs are more about converting people.
Spanking
MBs: Recently we stopped going to the MB church because the pastor preached, from the pulpit, that if you weren't spanking your child you were disobeying God.
Menno: Although most of us raised in the church were spanked as kids, it is changing. I am fairly sure that the current minister at my home church (that my parents still attend) doesn't spank his kids. There is a growing movement that questions spanking as appropriate (How can we expect to find non-violent conflict resolution in the world if we can't in the home), but I am sure it isn't universal by any means. It is probably more prevelent in the more liberal end of the spectrum though.
On a pure social note, in the area I grew up, there was the underlying joke that MBs thought they were 'More Betterer' than the other Mennonites, largely because the MBs tended to show their wealth a lot more than Mennonites did. (not that they were wealthier, but that they were more likely to have nicer cars and houses)
One of the things that is cool about the Mennonites and MBs is that they do come together for a lot of great causes. In general, they are able to put aside their differences to accomplish some pretty cool things, especially in the area of disaster recovery and helping the hungry and poor.
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