Need Some Advice For Master's Degree: Biomedical & Aircraft Design

As K says, aircraft design and biomedical appear to be poles apart. However, the curious thing is that both use the same fluid flow software for design.
Inner space is as aggressive as outer space. The same materials are used both in aerospace and biomedical devices. Aerospace fabrication processes are used for making implants. In fact, metal parts of one of the earliest artificial heart valves were being made at Pratt and Whitney in the US!
It is a small world after all.
I too love animals. In 1980, when I was asked to head a federal biomedical research institute in India I was in a tizzy since invivo animal experimentation is a prerequisite for biomedical device development. I do not believe that humans are superior beings except for being intelligent and able to control (and damage) the environment.
We set up one of the best animal (large and small animal) facility. Clean rooms, air conditioning and purified water and roomy cages were the norm. A team of veterinay scientists took care of the inmates on a 24 x 7 basis.

Coming back to your options.
Both aero space and biomedical are suitable. As I mentioned, you can even combine them in certain areas of biomedical implant design. Devices like artificial joints, heart valves, and implantable pumps require considerable engineering expertise in design. Before even starting any animal trials, rigorous in vitro testing for engineering performance under simulated physiological conditions is needed. Such tests require special machines for testing, developing of which itself is a major engineering activity.
Away from engineering aspects, animal testing as such can provide a fulfilling activity as I described above. One has to ensure that trivial experiments are not done, the animals get proper care and pain management is taken care of and such issues. However, it would mean that you must train in veterinary science.

There is a large area of biomedicine that is largely electronics and IT oriented. Even in this, there is a large component of mechanical engineering in designing the machinery and mechanisms.
Technolgy is a large component in current healthcare delivery. From the lowly wheel chair to the CNC controlled Cyberknife, technology is all pervading in a hospital. Maintenace of all such equipment is the job of the biomedical engineering services of a hospital. It is daunting, yet fulfilling. There is the inherent satisfaction of directly serving suffering humans, which is absent in design of devices sitting in a lab.

As you can see, it is a vast area. This is only a small overview. Please come back if you want any specific explanations on any of these or other queries that may arise.
Finally, we are not native English speakers either. I have worked for long in both Aerospace research and Biomedical Engineering R&D. I for one did not find my English giving any disadvantage in my work.

Thanks for everything. It really helps to open my mind and see different ways that I can take.
I really appreciate it.