Hey Mark - welcome to the forum! Happy to have you here!
All right, here’s my 2 cents. The chromatic harmonica is quite a different beast. I thought I wanted to learn chromatic, and bought one - it was quite expensive. I bought a book. I was all excited… for a few weeks…
So to answer your question how hard is it to learn chormatic: quite hard!
The diatonic is soooo much easier, and imho it’s more fun because you don’t have to know all the music theory stuff to play it, it’s smaller / more portable, and the joy of bending is such a big part of harmonica playing for me.
Now if you love jazz and Stevie Wonder’s stuff, go for it! But I’ll just stick with being happy I can play the Boogie On Reggae Woman solo’s, which are in 1st position on a diatonic, and if I’m not mistaken, Stevie’s only recording on a diatonic.
His playing on ‘I Guess That’s Why They Call It the Blues’ cannot be duplicated on a diatonic, and it sounds very daunting to me.
If you are very dedicated and have LOTS of time to devote to it I’m sure you could do it. Otherwise, perhaps consider choosing a different goal.
Hope that helps!