Hmm.....I think its best to clarify this with the admissions committee.
Here's my opinion...
The 250 words max. is the place for you state your goals and the reasons behind having such goals.
In the essay for Why Booth & Why Part-Time MBA, you will need to talk about the specific aspects of Booth that made it your preference, and also throw some light on why you choose to do a Part-Time MBA. Of, course you can relate some aspects of Booth to your short term and long term goals, but dont go beyond the surface level of mentioning your goals here in this essay. I think this should easy take 400 words.
For your reference, the FT MBA application has a 250 character limit on the goal, and NO word limit on the Why Booth essay, so it is expected that we talk in detail about our goals and how Booth is the right fit.