One option that might be overlooked is to omit the particle, as opposed to choosing "は" or "が". In this case, it would sound much closer to "は" than "が".
Personally, I would go with this third option, in most cases.
Nonetheless, I concur that context really is important when discussing Japanese. There are cases when one would sound correct, and the other incorrect and vice versa.
In what situations would you use [the former] in?
When the subject is made clear to be someone other than the listener.