Huh - I didn't read this as Chise choosing Joseph, I read it as her rejecting Elias (at least, as he is now). What happens with her and Joseph will be determined in the next chapters, all we know at the moment is that she's rejected Elias. We also need to remember that Joseph is possessing Stella, and Chise's not going to give up on getting Stella back any more than she'll give up on any of her other friends, so under the circumstances it makes sense for her to go away from Elias and stick with Joseph for the moment.
I mean, not that I think it's a good idea to go anywhere with Joseph under basically /any/ circumstances, but it's entirely consistent with Chise's personality, and it doesn't feel forced to me.