For K-8, I would recommend reading unless you intend to be a resource teacher and get an SPED credential. If you do not have a special education credential already you will need to get one too in order to use the Special Education M.Ed. With the push for Response to Intervention in elementary schools a teacher with an M.Ed. in reading would be an asset. There are also fewer people with an M.Ed. in reading. I would caution you to get hired before you begin the program though. Having a graduate degree makes you more expensive and may make it for you to find a job.
I would lean towards either special education or reading, these are positions that are not likely to get cut when funding becomes an issue. If you choose special education you need to be certain that you able to and want to deal with students with all types of disabilities. It truly takes special people to work in special education.
A masters in special education won't really enable you to be much more than a special education teacher with a masters degree. To be a special education administrator you will probably need a specialist degree or a doctorate. A masters of education in reading, however, would be very marketable in that federal mandated high stakes testing deals with reading. Many school districts will be looking for qualified reading educators to help with reading problems.
If I had to choose, I guess I would recommend a Masters in Special Ed. Considering the recession and the shrinking budgets of states and school districts, I think Special Ed is the most employable of the three fields. Having a Masters will also give you a better chance at being a Special Ed administrator or coordinator, which both pays better and is a critical job to have good people in.
In K-12 Education, I find that those with tech degrees becaome, by default, the IT person for the entire school, and it takes them out of the classroom. Schools are less able to afford their own IT help, so they use the talents of the people within the building, and if you love teaching and kids, that will take you away from them.
In my opinion they are all marketable, but most of the funding is usually going towards Educational Technology. Plus, you can carry that degree towards the Vocational and Community College level, or even best, you can become an Instructional Support Specialist in this area, which is a very weak area among teachers (not students). In an Ed Tech class you have minimal discipline problems because the new millennial learner is already familiar with most of the software, and you can use their schema in technology to build upon it and create great and cool projects. The kids never get bored in an Ed Tech class, and you will have probably a clientele of students that elects to be there instead of having to be there. Since you will likely be teaching an elective course, whoever goes to you is a techie, a geek, or a lover of technology. Hence, its a plus for you.
The only drawback is that the other teachers might see you as the "tech-nician" and will prob. end up asking you a thousand questions on how to fix their minor computer problems.
YET- The workload and stress that often come with SPED and Reading (especially since you will be teaching kids who are mostly struggling with a reading degree) is probably ten times thicker than the fun you could be having in an Ed Tech class. It is not an easy degree to get, but it will pay off in the end. I was there, and I regret ever getting out.