Hello and welcome to the forums.
I looked at both experiments, and they are very different. The ion engine project is more of a research project, and the fuel cell project requires building a fuel cell from a kit. The fuel cell project will take much longer to complete. I don't think either project will be a "winner' without significant modification by the student (which is a good idea anyway.) Judges don't tend to like either research projects or "kit" projects.
Questions to ask yourself:
- How much time do you have?
How much do you want to spend?
Do you prefer virtual projects or "hands-on" projects?
How can you extend the project into something more interesting?
If you can answer these questions, let us know what you decide.
Keith