Hi all,
While I still prefer "rsync" for transferring directories/files, for the last week or so I've been tinkering with Universal Control's "drag-and-drop" between two MBPs, and this seems to me to be the simplest method, even easier than AirDrop. While Universal Control is still in Beta, it should be available sometime this spring (northern hemisphere)/fall (southern hemisphere).
Solouki
P.S. For me personally, "rsync" is the fastest way and much faster than UC's "drag-and-drop" to transfer files (you don't have to use the GUI to drag the cursor across two displays as you do for UC; and for rsync an up-arrow/RET is all that is required to perform the differential transfer). Plus, if you are backing up or performing any type of differential update, rsync will transfer just those files that have changed since the last rsync, saving a lot of bandwidth. Rsync also operates bi-directional, and you don't have to be physically close to the other machine (rsync even works over the Internet).
Edit: And one more thing, UC's drag-and-drop only works between computers logged into the same accounts (same AppleIDs). "Rsync", on the other hand, allows one to transfer files between different accounts on different machines as well as different accounts on the same machine.