Either. We know that if you are trying to conceive, you shouldn't soak your testicles in hot water (Jacuzzis are out), but does wearing tight underwear create enough excess heat to adversely affect the sperm? It turns out that scientists have actually studied this question. Men who wear briefs have exactly the same scrotal temperature and sperm parameters as men who wear boxers - either one is fine.
Personal choice. Sorry, it's unlikely that underwear type has any significant effect on male fertility. Conventional wisdom has for years steered men towards boxers, although no convincing evidence ever supported this. Some small studies in the late 1990s looked at this, with no differences ever found. Routinely advising men to wear boxer shorts can't be supported by available scientific evidence.