Sorry for the length...
MRC's are virtually cake walks to join. Usually, you fill out a membership application and you're in after a brief phone interview and orientation training. The process of joining a DMAT is not nearly as straight-forward. Because, upon joining a DMAT, you are considered to be an interim federal employee, you need to meet the standards that the US Dept. of Health and Human Services (HHS) have set for employment. In this case, for an EMT-B, this includes one year of experience. After having a phone interview, you need to go to a HHS field office to get finger printed. Then you need to pass a rather lengthy background check before you are offered employment. More form to fill out, etc., etc., etc.... It's a federal job, so there is a lot of red tape to go through.
The real question regarding the two organizations are how much time are you willing to volunteer and what do you want to be doing.
MRC's generally provide vaccine clinics and serve in other public health initiatives on a voluntary basis to areas in and around their locality. These organizations are especially aimed at registered nurses, although they happily accept most any volunteer that comes their way. At mass disaster sites, MRC's are assigned primarily to first aid duties (perfect for an EMT). They are sometimes also used to back-fill DMATs. Despite the fact that they may deploy to a federal disaster site, they do so without pay. MRC's are, at this time, strictly voluntary.
DMATs are assets of the National Disaster Medical System and are used to deploy to disaster sites throughout the nation; New Jersey's DMAT-1 was actually sent to Haiti last week to aid in the disaster recovery efforts there in lieu of their earthquake. DMATs create M.A.S.H. like field hospitals and are virtually ED's that can deploy throughout the country (and, apparently, the world, as is the case for NJ DMAT-1 and other such DMATs). Unlike the MRCs, DMATs positions are paid positions. Members are paid for their deployments. They are also protected against losing their full-time employment, as their deployments are considered to be out of military necessity. This is not to say that everything that a DMAT does is for pay; often times, a DMAT will deploy to events in their locality, not on the request of the federal government, but as a community service. In such a case, those who deploy with the organization do so on a volunteer basis. Additionally, it should be mentioned that if a unit is deployed, a member is not required to deploy with the unit. If you can get away from work, the wife and kids, etc., then you can go; if not, others will fill your spot. DMATs also require specialized training: In addition to ICS 100 and 200 and NIMS 700 and 800, regular meetings are compulsory, as is a weekend (overnight) training. A member is not paid for the meetings and overnight training.
I hope this is of some help to you. I am just getting on a DMAT myself (stuck in the red tape). I chose the DMAT over the MRC because of the greater diversity of work and the fact that I can't afford to be away from my job without pay for two weeks. Either option is respectable. Good luck!
