Technically they are the same thing as bi just means hetero (different) and homo (same). It doesn't mean only cis men and cis women. Historically it has included trans and non-binary people. Everyone has preferences, even people who refer to themselves as pan. Just because you aren't attracted to, say, trans women, it doesn't mean you are kicked out of the pan club. Likewise, being attracted to more than cis people doesn't mean you aren't bi. Basically if you are pan you are bi because pan falls under the bi umbrella.
I usually just say I am bi/pan to cover people who think they are completely separate things. I prefer bi, and sometimes I feel iffy about pan just because it tends to further the idea bi people are attracted to cis only, which tends to give it a negative slant.