Those are all really nice machines and it's not an easy choice. I'd wait for Dell to refresh the XPS 15z with Ivy Bridge before picking it up. Not sure when they'll update the XPS 13 with Ivy Bridge. The XPS 13 has the winning combo of being the most portable and durable. The XPS 15z is very powerful-- probably more than you need for the tasks you listed. We haven't gotten the Spectre XT in for review yet, so I can't say about it except that it looks great on paper. The Envy 14 is going away and thus isn't a top pick.
-------------------- Lisa Gade Editor in Chief, MobileTechReview