Definitely a divisive topic. My understanding is that bisexual is attraction to two or more genders (of any nonzero amount of attraction) and that pansexual is equal attraction to all genders. Bisexual is an umbrella term and pansexual is a specific type of bisexual.

However, how people decide to label themselves is a bit tricky. Some folks consider themselves pansexual, and the bisexual label does not apply. Other pansexual folks will say that bisexual also fits. There are even more labels, like omnisexual (attraction to all genders, but the level of attraction between them is not equal), that exist but aren't in as wide of use. It's not really my place to say whether these labels are valid or if these folks are using them right, but this is what I have observed over time.

If I needed to make a multiple choice form for collecting sexuality information, I would likely have straight, gay, bisexual, asexual, other and prefer not to say, with each option allowing notes if wanted. This would still allow for people to choose a label that fits them and be more specific if they preferred. (A pansexual person, for instance, could choose bisexual or other and further specify if they so chose.)

Oh no! Nuance?? We can't have that! Put those people in the same box, ASAP!!

Fucking hell, just let people have their distinctions.