OK, just wrote a long responde, then saw i misunderstood your question. Great!
Anyway, I recently went from a 2010 MBA to a 2011 and I used Time Machine. To me, the stuff that matters are some applications, iTunes library, DropBox (excluded - already in the cloud) and some custom stuff like launch agents and tweaking.
Option 1: So, if I was in your shoes, I would setup the new machine as a new. Then copy the stuff you need over form the old one. Then you'll be able to place the static data on the big external drive and I think its healty to setup and clean your system from time to time. This is a good solution if you have not tweaked your system to bits with custom launch agents and such. This depends on how custom your current machine is!
Option 2: Take your new external disk, set it up as a Time Machine. Then exclude the big files and folders form your old machine. This includes Steam Apps, DropBox, iTunes library and such. Use a app to determine the biggest folders. Then, restore the new machine from that TM backup.
When your system is up and running, then just copy the rest over. The TM backup restore can be slow is my experience.
ofcourse, take the backups you need. With a big 2 tb disk, you can even clone the whole hdd of the MBP just in case. This post got long as well...
