Optimal Mac setup for web designer/developer?
I believe that your third option will be easiest to work with, since you have the mobility that you desire (i.e., 13" model), which can be connected to the 27" display when in the office. In particular, the system should be responsive when hooked to the external display, even with all the windows and applications that are running simultaneously.

One thing I'd suggest is to get 13" model that can be upgraded to 1TB storage. You'll be getting years of usage out of the macbook, and you don't want to be starved for storage when undertaking new types of projects in the future, such as the iOS development that you mention (not to say there is huge overhead with iOS, just that you want to be prepared for something that wasn't anticipated).