It really is just a preference thing, like there isn't any real technical difference (not to my knowledge, at least) about which key you start in, a lot of it is dependent on what sounds best to you. The only reason you should start with C is that that's probably the 'default' for most lessons you'll get online or in books, and it might prove moot to look for beginner harmonica lessons in B online.

Like, it's the type of thing everyone does because everyone else did it, so yeah, start with C, just because finding tutorials be easier. But yeah no, I'm not a harmonica expert, I'm interested in like, percussion, toy instruments and extended techniques-- basically horrible music that babies make. I just figured it should be known how great it sounds to BLOW SLOPPILY ON A D (even tho you'll be making it harder for yourself to learn if you don't start with C).