Is it appropriate to ask a prospective PhD advisor whether working for them would be good for my career?
Could asking that question upset the advisor...


You have a legitimate concern. If advisor 2 is angry that you brought up a legitimate, important concern, you're probably better off working with advisor 1. 


  ...and negatively influence my chances for later becoming a postdoc in their group?


It is far too early to start worrying about specific post-doc options.