The future of anesthesia (based entirely on what I've read.. no experience here at all) is pretty unclear. Many hospitals are picking up more CRNAs in order to lower costs, however when a CRNA works independently they are pushing into the area of practicing medicine. There's legislation going around that is either going to open up the CRNA field by allowing independent practice or limit CRNAs to require MD supervision (whether it's an MDA or any other MD is another issue).
In most cases, both MDA and CRNAs can perform anesthesia services in the same quality, however an MDA does have medical school under his or her belt, so in some complicated cases, the MDA may be more able to handle the case.
MDAs make significantly more money, but they also have to carry a tremendous liability insurance
. The future salary is also unclear due to Obama's healthcare plan that is in the works. You can expect salaries for both to decrease.
One MDA I have spoke with about careers told me that if I'm absolutely certain that anesthesia is the only thing I want to do then I should save my time and money and become a CRNA, but he also said that if I have the slightest doubt I might want to pursue another healthcare/medical field.. then I should go to med school and figure out my specialty along the way.
Hope that helped somewhat.