Level drain. There are spells that fix level drain completely, sometimes even putting you ahead on XP. Neither death nor rust monster attacks can be wholly reversed (especially pre-3E, where death costs you a permanent point of Con).
Of course, it depends somewhat on what level I'm at. If I'm a 1st-level fighter, my answer will be the rust monster, since mundane gear is relatively easy to replace and level drain will kill me. But in general, assuming a friendly cleric of sufficient level, I prefer level drain.