I think for an HP one it would be easier to be a first year. Choosing any year would be a challenge. First and foremost, you'd have to expand your character's history section to include what happened the first x years they were at Hogwarts. What makes this harder is that there could be certain things that did happen in previous years that the GM may wish to hold back. Also, I don't think wizards/witches are allowed to transfer as new students or from one magical school to another, but I'm not entirely sure about that, which rules out that option. Finally, if you're any year besides first year, you'd probably have a group of friends or at least a good few, unless you're a social recluse or psychopath or something. It'd be hard to interact with first years, and you'd have a lot of NPCs on your hands.
Idk. Just my own thoughts.